Hey apollo--
My 2.5cents are that 1, you shouldn't release fish back to the wild, no matter how long you've had them in captivity. Their momentary exposure to your already captive Redbreast could have introduced them to a pathogen they'll now take back to the stream. I won't harp on it as I'm sure you had good intentions but it is a safe measure we follow in fishkeeping.
2, I think your 40B is owned by the Redbreast. What I mean is that s/he has established the entire tank as his/her territory. Let's say HE for the sake of the argument, is coming into breeding season. He will not tolerate any other fish coming into his potential nesting site. That is usually where folks run into issues with sunnies. Redbreasts get very large and it is unlikely you'd be able to have any others in there with him. *IF you are inclined to try more, I would recommend taking him out and rearranging the tank before reintroducing ALL of the fish at the same time. Breaking up the 'line of sight' (by having a lot of driftwood/large rocks) would help lower the aggression. (Think; they can't be mad at what they can't see). I would not recommend more than 4-6 depending on their size. But then again, you have to remember how big they are going to get in the long run and potential breeding issues down the road.
3, so let's try to fix your current situation. Dithers. Small, agile, quick shiners is the way you want to go. You can look up what species you have available in your area and try to collect some. You could probably work with a good dozen for starters and see how they do.
Keep in mind that whatever option you try, you absolutely should not release them back to the wild. So be prepared to move fish that are getting picked on to another tank. Or give them to someone else. I hope something works out for you and your lone Redbreast. Stay safe!