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I don't have first hand experience with GBRs mixed with BRs but I do have GBRs and electric blue rams in the same tank. They should be fine, they are basically color morphs of the same fish, much like how a marble angelfish, black angelfish, and koi angelfish can all live in the same tank. Still be sure you have more or equal females than males. I have 2 male electric blues and 2 female GBRs
It depends on the size of the tank and the amount of cover you have, but yes, given enough space and territory, there is nothing preventing you from housing them together.