I love box turtles too. I finally just recently got my first box turtles. I vote for 3 toed and Eastern. I think they are both beautiful. If I could have Easterns here I would have them too.
If you already have a gulf coast you should just get some more of them and look for ones about the same size as yours.....if your trying to keep them in the same pen. If your going to keep them separate then you can get whatever you want. I have (3) three toe boxies and love them.