I put a 3 inch one in my 125 that had a 12 inch one in it already. Now the small one is probably 5 inches. They hang out next to each other a lot and have never fought.
I have 2 ornates. both are 7"-8"inches. One has a thinner body than the other. and the bigger one bullies him whenever they get close. that's why his tail doesn't grow completely. I dont know exactly why, is this domination?
A friend has two about 12" that have been grown out from about 3" together and they get on fine. I think mixing two different sized ones could be a different story though.