In Fardh Salaat, it is Wajib in the first 2 Raka'at to recite Surah Fatiha and another Surah after it. In the remaining Raka'at, reciting Surah Fatiha is Mustahab. There is no command about another Surah.
Apart from Fardh, be it Sunnah, Witr or Nafl Salaat, it is Wajib to recite Surah Fatiha and another Surah in all 4 Raka'at. Thus, another Surah should be recited after Surah Fatiha in all 4 Raka'at of Sunnah Salaat.
Saying Aameen after Surah Fatiha
Saying “Aameen” after recitation of Surah Fatiha is a Sunnah. If someone forgets it and the Imam recites another Surah after the Surah Fatiha, then the time for saying “Aameen” has passed and it cannot be compensated. Sajda Sahw does not become Wajib on missing a Sunnah in the Salaat.
Thus, the Salaat of such a person will be correct, but he should be mindful of the Sunnah the next time. It is given in Durre Mukhtaar, Vol. 1, Pg. No. 351:
Translation: It is Sunnah to say “Ameen” after recitation of Surah Fatiha.
Again, it is given on Pg. No. 350 of the same book:
Translation: The Salaat is not rendered invalid on missing a Sunnah and neither does Sajda Sahw become Wajib. However, doing so is not reasonable.
Reciting another Surah before Surah Fatiha
If any person recites some other Surah before Surah Fatiha then that person should first recite Surah Fatiha and then some other Surah. If that person has recited even 1 complete verse, then Sajda Sahw is Wajib (compulsory) on that person, as reciting 1 verse of the Holy Quran is considered as Rukn (article) of Salaat and if recitation of Surah Fatiha is delayed to the extent of 1 Rukn, then Sajda Sahw becomes Wajib. If less than 1 verse is recited, then Sajda Sahw is not necessary. As given in Radd Ul Muhtaar, Vol. 1, Pg. No. 340:
Translation: If somebody mistakenly recites at least 1 verse of some other Surah before Surah Fatiha, then that person should first recite Surah Fatiha, then some other Surah and perform Sajda Sahw. Sajda Sawh has become Wajib because of delay in recitation of Surah Fatiha. A slight delay, which is less than performance of 1 Rukn or less than 1 verse of the Holy Quran, is excused.
Going into the Ruku after reciting Surah Fatiha only
Ruku means to bend. The least limit of Ruku is to bend so much that the hands touch the knees. If someone bends less than this, then this is not called Ruku and that person is considered to be standing (Qiyam) only. That person can stand straight and recite another Surah (Dhamm Surah). There is no need to perform Sajda Sahw (Sajda of forgetfulness). Regardless of whether it is Fardh, Wajib, Sunnah or Nafl Salaat.
As given in Fatawa A'alamgiri, Vol. 1, Pg No. 70:
If someone bends to the extent that the hands just touch the knees, even then they can come back in Qiyam, recite another Surah and perform Sajda Sahw. As given in Raddul Muhtaar, Kitab Us Salaat.