No, I think would be fine. You won't be able to add any more fish though, because that'll be about max bioload for that size tank. And you'll need plenty of rockwork so they can hide from eachother any time they need to. One less fish would probably be a little bit safer as far as aggression goes. A single firefish is likely to get harrassed in a tank on its own. On that topic, something to think about is an arrangement that makes your fish the happiest. For example, firefish are much more content in groups of 3 or more just like clowns are less aggressive when kept in pairs. So if you really want firefish, maybe get a small school of them instead of having singles of a bunch of fish. Just something to think about. There a re endless options.
If you do wind up keeping that many fish in your tank (I'm not one to talk since I always have max. bioload in my tanks as long as everyone gets along), make sure you do plenty of water changes. I do a 25% water change on every one of my tanks once a week whether or not they need it. It's just a good habit to get into.
Can't wait to hear what you decide on. When are you starting to set up your tank?