I have 3 red firefish in a 125. I think 2 would be ok in a 75, but I'd cover that tank with egg crate before I got them, even the overflow box. They are notorious jumpers. I added 3, I already had egg crate on the back but not over the overflow, and the next morning one was in the overflow box, another in the sump, only one was left in the display tank.
They can be shy. Mine are doing fine with 5 chromis, 4 PJ cardinals, a clown, a sailfin tang, and a mandarin. All my fish are pretty mello. Some people report their firefish are scared to eat because of other, active fish, and starve to death.