Your Chromis and Firefish will very likely start picking each other off at some point. Chromis slowly whittle down to 1 in all but the LARGEST tanks (I'm talking the tanks that make yours look like a mini sump size here), and for whatever reason firefish will get along just fine and one day BOOM, they flip out until there's only one.
So be cautious of that happening!
If you plan to introduce any other fish (I would!) get an acclimation box so they can get used to one another. There's a lot of awesome wrasses out there that you could choose from, and many of them will be picked on by the Melanurus as he continues to grow. Having an acclimation box allows them to get used to one another and establish a hierarchy without things getting 'physical'. VERY useful tool, especially if you start getting expensive wrasses.
Otherwise, flasher wrasses would be a good addition. They look nice and they shouldn't have problems with anything in your tank -- Wrasse included.