I wouldnt see an issue with that as long as there are plenty of spaces to hide and you only get one gramma in a small tank. Firefish are pretty peaceful fish from what I have read and are a good community fish, but just as grammas they are quite often tough to keep as pairs. As I said, give good current, plenty of hiding space and you should be ok. What size tank is this?
If you are trying to put them in a 10 gallon I would probably say no, but if this is 30+ you would probably be ok.
--Andy