Over the years I've tried red breast a half dozen times, without any success. For me they have been one of the most disagreeable Lepomis, always either picking on or being picked on. I have not tried one as alpha, so I lost all of them to aggression.
Aggression is not a static thing, it is an ever changing dynamic. I'm surprised you have not had any trouble with your current setup between the bluegill and crappie. My son recently lost a 5" crappie to aggression to a 3.5" pumpkinseed. They have been perfectly fine together all summer, then one day last week we find a dead crappie from a bad beat down.
Lepomis tend to be most aggressive in their "teenage" stage, from after sexual maturity to being full grown. After that they tend to mellow out a bit when the stop grown longer, and start growing fatter and deeper. 2 years ago my Pumpkinseed and Bluegill wiped out the rest of the tank except the alpha Green, and they had to be separated or one or both would have also been lost. Today they are fat and lazy, and so peaceful that they can be kept together with goldfish.
If you do decide to try a redbreast, then I would suggest only 1. Lepomis tend to be more tolerant of other species and most aggressive towards their own.