Eight, at least that's how many I can see, would be a good number of mature gibberosa in a seven foot tank. That's not to say you couldn't do a few more, especially since they are young. I think another quad of juvies would be good at this stage. That would give you some flexibility later on if you needed to start removing males. I would recommend it
I think, I count seven in there now. In a seven foot tank, at their current size, I think you could add a few more. Reaquascape a little when you introduce the new fish.