Hi Jim,
How big are your older rainbows in comparison with your juveniles ? If the
size differential is too large, the bigger ones may seek to make a meal of
the smaller ones. I recently released 6 lacustris and 3 praecox about 1 to
1.5 inches long into my large community tank of trifasciatas (about 4-5
inches), and the trifasciatas chased the lacustris and praecox around the
tank. So much so, that some of the the juveniles died of stress a few days
later. I had to scoop the remaining ones out and move them to a smaller
tank to allow them time to grow out. Even if the larger bows don't
terrorize the smaller ones, in my experience, the smaller ones have their
growth retarded due to being outcompeted for food.
Chen Leong