A standalone word of a single letter should have a meaning
As for the examples of letters you've quoted none of them is a standalone word that has a meaning in the Arabic language. And the qur'an commentators gave a couple of explanations on the use of these letters at the beginning of some of the surahs of the qur'an. So the they are clearly a matter of assumption not a matter of clear meaning and shouldn't therefore count as stand alone words.
Imam ibn Kathir says in his tafsir about the letters at the beginning of the usrahs:
The wisdom behind mentioning these letters in the beginning of the Surahs, regardless of the exact meanings of these letters, is that they testify to the miracle of the Qur'an. Indeed, the servants are unable to produce something like the Qur'an, although it is comprised of the same letters with which they speak to each other. This opinion was mentioned by Ar-Razi in his Tafsir who related it to Al-Mubarrid and several other scholars. Al-Qurtubi also related this opinion to Al-Farra' and Qutrub. Az-Zamakhshari agreed with this opinion in his book, Al-Kashshaf. In addition, the Imam and scholar Abu Al-'Abbas Ibn Taymiyyah and our Shaykh Al-Hafiz Abu Al-Hajjaj Al-Mizzi agreed with this opinion. Al-Mizzi told me that it is also the opinion of Shaykh Al-Islam Ibn Taymiyyah. Az-Zamakhshari said that these letters, "Were not all mentioned once in the beginning of the Qur'an. Rather, they were repeated so that the challenge (against the creation) is more daring. Similarly, several stories were mentioned repeatedly in the Qur'an, and also the challenge was repeated in various areas (i.e., to produce something like the Qur'an). Sometimes, one letter at a time was mentioned, such as Sad, Nun and Qaf. Sometimes two letters were mentioned, such as
﴿حـم ﴾ (Ha Mim) (44:1)
Sometimes, three letters were mentioned, such as,
﴿الم ﴾ (Alif Lam Mim (2: 1))
and four letters, such as,
﴿المر﴾ (Alif Lam Mim Ra) (13:1), and ﴿المص ﴾ (Alif Lam Mim Sad) (7:1).
Sometimes, five letters were mentioned, such as,
﴿كهيعص ﴾ (Kaf Ha Ya `Ayn Sad) (19:1), and; ﴿حـم - عسق﴾ (Ha Mim. 'Ayn Sin Qaf) (42:1-2).
This is because the words that are used in speech are usually comprised of one, two, three, four, or five letters.''
Every Surah that begins with these letters demonstrates the Qur'an's miracle and magnificence, and this fact is known by those well-versed in such matters. (Source qtafsir)
A single letter word in Arabic is possible
In Arabic a single letter word can only exist as a reference to another word which has been quoted in prior. So this means that it can hardly apply to those words at the beginning of a surah if we consider each surah as a closed entity. Therefore typically you may find them in first place beginning from the second sentence (or second part of a sentence) in an Arabic text. As it doesn't even make sense in another language to start a sentence with" And … " or "That .." or "So …" or "Then…" without prior information. One could say that these kinds of words depend on other words, as their meaning is in the reference to another word. Note that it needs certainly a good basis in Arabic language to distinguish these single letter words from a letter that belongs to another word. For example:
The letter "waw" in the form "وَ" will be an example I'll quote later, but we can't just pic it from a word as one must distinguish between i.e.:
وَيْلٌ (Woe) like in (45:7)
ووَيْل (and woe) and
فَوَيْلٌ (So woe) like in (2:79)
As a hint for the given examples I'll mark the relevant words referring to the single letter words I'm discussing here in Arabic in all examples.
I'll mark in bold letters the English synonym that refer to actually discussed letters (words).
While you may find in italic those who are referring to other letters (words) that are discussed in this answer.
I hope this doesn't create confusion!
The "waw" as a single letter word
While a simple letter which often finds use in the qur'an and Arabic language and has a meaning is:
وَ (transliteration: wa)
which usually means in the given cases "and" like in:
إِيَّاكَ نَعْبُدُ وَإِيَّاكَ نَسْتَعِينُ
It is You we worship and You we ask for help. (1:5)
صِرَاطَ الَّذِينَ أَنْعَمْتَ عَلَيْهِمْ غَيْرِ الْمَغْضُوبِ عَلَيْهِمْ وَلاَ الضَّآلِّينَ
The path of those upon whom You have bestowed favor, not of those who have evoked [Your] anger or of those who are astray. (1:7)
As in the last example و is used with a negation a better translation is:
The way of those on whom You have granted Your grace, not (the way) of those who earned Your anger, nor of those who went astray (Source qtafsir)
"wa" moreover could also initiate a vow like in:
فَلاَ وَرَبِّكَ لاَ يُؤْمِنُونَ حَتَّى يُحَكِّمُوكَ فِيمَا شَجَرَ بَيْنَهُمْ ثُمَّ لاَ يَجِدُواْ فِى أَنفُسِهِمْ حَرَجاً مِّمَّا قَضَيْتَ وَيُسَلِّمُواْ تَسْلِيماً
But no, by your Lord, they will not [truly] believe until they make you, [O Muhammad], judge concerning that over which they dispute among themselves and then find within themselves no discomfort from what you have judged and submit in [full, willing] submission.
By the fig and the olive (95:1)
Single letter words with "fa'" and "kaf"
There are also other letters that have a specific meaning, but are by default attached at the beginning of the corresponding words like:
فَـ (transliteration fa)
It means in the given cases:
"then" or "and then" or "and" or "so then" or "so" or "therefore" ... it usually expresses a kind of continuation.
فَذَلِكَ الَّذِى يَدُعُّ الْيَتِيمَ
For that is the one who drives away the orphan (107:2)
So woe to those who pray (107:4)
فَصَلِّ لِرَبِّكَ وَانْحَرْ
So pray to your Lord and sacrifice [to Him alone]. (108:2)
فَسَبّحْ بِحَمْدِ رَبّكَ وَاسْتَغْفِرْهُ إِنَّهُ كَانَ تَوبًا
Then exalt [Him] with praise of your Lord and ask forgiveness of Him. Indeed, He is ever Accepting of repentance. (110:3)
Another example is the letter (kaf):
كَـ (transliteration: ka)
which usually means (when added at the beginning of a word) "like" or "similar to" etc. like in:
يَوْمَ يَكُونُ النَّاسُ كَالْفَرَاشِ الْمَبْثُوثِ
It is the Day when people will be like moths, dispersed, (101:4)
وَتَكُونُ الْجِبَالُ كَالْعِهْنِ الْمَنفُوشِ
And the mountains will be like wool, fluffed up. (101:5)
فَجَعَلَهُمْ كَعَصْفٍ مَّأْكُولِ
And He made them like eaten straw. (105:5)
Note that at the end of the word this letter may be the pronoun "You" like in surah al-Kawthar (108) in the words:
But these kinds of letters don't count as a standalone word in Arabic they are considered as references (in the first case even due to conjugation as the first example is a verb).
Ba' , ta' and lam as standalone single letter words
Again these letters are attached at the beginning of a word:
بِـ (transliteration: bi)
Is usually used to initiate a vow or expressing a relative position, sometimes it may sound like a place holder form a non-Arabic speaker (therefore I might need to add additional translations to the given examples): It usually means "to" or "by" or "with" or "in" depending on the context. Like in:
بِسْمِ اللَّهِ الرَّحْمَـنِ الرَّحِيمِ
In the name of Allah , the Entirely Merciful, the Especially Merciful. (1:1)
وَمِنَ النَّاسِ مَن يَقُولُ ءَامَنَّا بِاللَّهِ وَبِالْيَوْمِ الأْخِرِ وَمَا هُم بِمُؤْمِنِينَ
And of the people are some who say, "We believe in Allah and (in) the Last Day," but they are not (from among the) believers. (2:8)
قُلْ أَعُوذُ بِرَبّ النَّاسِ
Say, "I seek refuge in the Lord of mankind, (114:1)
أَرَءَيْتَ الَّذِى يُكَذِّبُ بِالدِّينِ
Have you seen the one who denies (in) the Recompense? (107:1)
تَـ (transliteration: ta)
It appears in the qur'an only as an initiation of a vow in the form:
تَالله (see here most if not all occurrences)
like in the verses:
قَالُواْ تَاللَّهِ لَقَدْ عَلِمْتُمْ مَّا جِئْنَا لِنُفْسِدَ فِى الاٌّرْضِ وَمَا كُنَّا سَـرِقِينَ
They said, "By Allah , you have certainly known that we did not come to cause corruption in the land, and we have not been thieves."
لِـ (transliteration: li)
it usually appears in the qur'an to express an explanation or to explain a cause it usually means "to" or "in order to" or "due to" or "because of" etc. like in the verses:
الْحَمْدُ للَّهِ رَبِّ الْعَـلَمِينَ
[All] praise is [due] to Allah , Lord of the worlds -
فَالْتَقَطَهُ ءَالُ فِرْعَوْنَ لِيَكُونَ لَهُمْ عَدُوّاً وَحَزَناً إِنَّ فِرْعَوْنَ وَهَـمَـنَ وَجُنُودَهُمَا كَانُواْ خَـطِئِينَ
And the family of Pharaoh picked him up [out of the river] so that he would become to them an enemy and a [cause of] grief. Indeed, Pharaoh and Haman and their soldiers were deliberate sinners.
Note that it is a complex topic and there might not be a general agreement on whether these or other letters count as standalone words. And which of the accepted cases actually are standalone words or the letters might be considered as parts of a word and can't be "detached" from them (even in our thoughts).