1 - In sajda we must say "سبحان ربي الأعلى" (subhana rabia al a'ala) once at least, that is wajib (mandatory) in each sajda and it's sufficient. The second, the third time and more than that are mustahab or sunnah. After that you can make dua as you like and in each sajda you want not only in the last one. And this Du'a is Mustahab because The Messenger of ALLAH (PBUH) said:
"وعن أبي هريرة رضي الله عنه أن رسول الله صلى الله عليه وسلم قال: "أقرب ما يكون العبد من ربه وهو ساجد، فأكثروا الدعاء"
"A slave becomes nearest to his Rubb when he is in prostration. So increase supplications while prostrating." Narrated by Muslim.
2 - And that is in fard(mandatory) and in sunnah because no Dalil (directory) to customise the mandatory or sunnah prayer.
3 - And it's recommended to make the dua which the Prophet have made.
4 - It's no recommended in the Jama'a Salat because
The Apostle of Allah (PBUH) said: When any one of you leads the people in prayer, he should be brief for among them are the young and the aged, the weak and the sick. But when one of you prays by himself, he may (prolong) as he likes.
5 - There is another time to make Du'a in salat. It's after Tashahod in the Hadith:
"ثُمَّ يَتَخَيَّرُ مِنَ الْمَسْأَلَةِ مَا شَاءَ"
The whole hadith is narrated by Muslim and reported by Ibn Mus'ud:
While observing prayer behind the Messenger of Allah (ﷺ) we used to recite: Peace be upon Allah, peace be upon so and so. One day the Messenger of Allah (ﷺ) said to us: Verily Allah is Himself Peace. When any one of you sits during the prayer. he should say: All services rendered by words, by acts of worship, and all good things are due to Allah. Peace be upon you,0 Prophet, and Allah's mercy and blessings. Peace be upon us and upon Allah's upright servants, for when he says this it reaches every upright servant in heaven and earth (and say further): I testify that there is no god but Allah and I testify that Muhammad is His servant and Messenger. Then he may choose any supplication which pleases him and offer it.