Praying sunnah when/while the Jama'ah has started
It is totally haram to pray anything sunna or tatwaoo' when the iqama has been called. when the Iqama has been called for any of the five Fard prayers, and one is praying sunnah or any other salat one must stop and go join the Jama'ah. The Prophet (peace and blessings be upon Him said):
إِذَا أُقِيمَتِ الصَّلاَةُ فَلاَ صَلاَةَ إِلاَّ الْمَكْتُوبَةُ
When the Iqamah for prayer is said, there is no prayer
except the prescribed prayer
It is also narrated that the Prophet saw a Man praying while the Iqama was being called, so the Prophet asked:
Are you praying Subh with four Rak'ahs?
Praying Sunnah or other voluntary prayers while traveling
You do not have to make sunnah or any other voluntary prayer/tatawoo', Source. But if you want you can pray tatwoo'/voluntary prayer and witr while your riding. For the Prophet (peace and blessings upon HIm) used to make these prayers on the beast He was riding, but He never made any Fard salawat on it Source.