I had 3 neon gobies and took one out, because it wasn't doing well. They got along fine before I moved the third, but it started beating up on the other two once I reintroduced it. It wouldn't let them eat and one eventually died. I was mad that the two didn't just move to the other side of the tank to get away from the bully. It's a 125gal for goodness sake!
So if you add another one, add it to the other side of the tank. I think they will either fight or pair up.