Your big problem is tank size. 30g and under isn't big enough for green sunfish to establish territories and coexist peacefully with each other and other fish.
55g might be ok, but bigger would be better. They seem to keep to themselves given enough room and good aquascaping. None of mine have ever bothered other fish unless picked on themselves. The one I have now in my 125g is fully mature and he's peaceful unless another fish messes with him, then you get to see what he can do

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It helps to have several fish to spread the aggression also. With just 2 fish there's more chance for one picking on the other.