Momot and Momotov are essentially the same. Momot would be the original root (Young). The next generation may have "ov" added to the ending signifying belonging, in the same way English Steven may become Stevens in the next generation. Thus, you should search for both. Momotov of course should be your priority. Luckily, your search will be simplified due to Russian tradition of giving the farthers first name as a middle name to the children. That should give you one extra generation deep.
In the last 10 years many Kossak Regiments got reinstated. Kuban' Kossaks have an organization that may be of a great help to you. Also there is a archive site that may be of a great help to you:
http://www.kubanarchive.ru/#
This site is in Russian exclusively, however you should not have a great problem writing in English or Portuguese, however I would advise you to list the language and encoding in English in the first few lines of you message. Using Unicode maybe the best route. Удачи Вам Госпожа Петрова.
SN :)