I usually wait until the babies are at least 2.5 to 3 in in size before they are released into the pond with the larger ones. The reason for this not just competition with the larger turtles, but less chance of the little ones being eaten by a frog that may stop by. If you are keeping them in a small enclosure, I would not put them together until they are half the size and weight of the largest one they will be with. Len
they should be roughly the same size, maybe up to half their size, the little ones will have to fight for food more than the bigger ones, you can get some more, but keep them separate until they are bigger, plus the larger RES may see them as food also.