I do this about every year with feed trained largemouth bass to get them to a size where they will not be forage for the larger bass in my pond when I liberate them into the pond. I put 5 to 7 inch largermouths in the cage in spring and they are a good 10 to 12 inches in a few months.

I use larger cages though of about 7 feet by 7 feet by 4 feet deep for 100 bass. I have a market for larger bass and sell them on a regular basis. See author's homepage.
I experimented with feed trained crappies in a cage last year with little success.

As far as hybrids that does not surprise me that the growth is erractic. I'm not impressed with hybrids and I would not have them in my pond. I would try regular bluegills, and you will probably find more uniform growth, or you could do some grading at some point as fish do have a pecking order like chickens.

Redear allegedly do not take commercial feed as you allude to but there may be exceptions.

If pigs could fly bacon would be harder to come by and there would be a lot of damaged trees.