I know how hard it can be when we want a specific kind of fish, but if I were you, I personally wouldn't add anything more to that tank. The fish in there now will need room to grow, and as the angels get larger they tend to get very aggressive and territorial.
Balas are a bit slower growing, but a good thing you've decided to move it out... before a problem happens.
The approach I guess I would suggest for you would be to think about an upgrade, and once that is done, then either make it a big enough tank to hold more fish, or use the 55 for the other smaller fish that you want.
When I describe this sort of thing to a customer, I usually compare it to a child outgrowing their clothing... and it can happen just that fast.
I'm sorry if that isn't the advice you wanted to hear, but I am honest... it really isn't worth the problems that will come from an overstocked tank once the fish start to grow more. If you can't afford to upgrade within the next 6 - 8 months, the tank is best left as is until the upgrade happens. To add more fish now would be inviting a world of issues.