I have heard that firefish are the first fish you need to add to the tank. I also heard that you can't have more then 2 and only 1 will live.
I have 2 firefish that I bought from the same tank, they were the first fish I added and I have had them for a year. So mine get along. If you add 2 more to the 3 I would say based on what I know they will fight but who knows. It's a smaller tank, so I would say no don't add them. They are almost impossible to catch if you do add them and want to take out lol SO FAST!
These first 3 are from the same group purchase and are still young and, so far, getting along fine. Thanks! I won't add anymore. I'll keep an eye on them...