[Marcello]

"Ukranians fighting in the conflict?"

Learning how to use a T-55 isn't like learning how to use a high performance jet. A SU-27 is a slighty more intricate matter than a 50's soviet tank.If you do not have the luxury of being able to wait for years for the training of pilots and technicians you will need to hire mercs.
During the Cold War it was common practice for the USSR and eastern countries to send pilots to fight for their clients, even for wars in which they were not particularly involved. Now it is still happening, for cash only.