the first fish I got for my 38 gallon tank back when i first set it up (after it cycled, of course) was a school of firefish. I got 3 of the normal ones, and they all did fantastic together. it was really cool looking actually, watching them in a loose formation in the tank "clicking" their dorsal fins into that upright position at odd intervals. It was really sweet. No fighting or anything.
Then one of my LFS got in a purple firefish for like 20 bucks, which is a very good deal, so I bought it. Added it to the tank, and the other firefish started kinda messing with him. So I caught him, and got one of those small animal plastic cages with the slotted lids...I think they used to call them "kritter keepers" and put the firefish inside that and submerged that in the tank. After about 3 days I let him out, and he joined the school with no incident. So if you decided to mix, I would follow this procedure that I used with great success.
Of course, all my firefish died about 2 weeks later when the power went out for 2 days and my tank plummeted to like 65 degrees. But had that not happened, I'de probably still have them all today....actually talking about it again makes me want to go buy some lol. toodles