Fire gobies (Nemateleotris species) are fine in groups and how I always keep them. Even in the shop we had groups of 10-20 and they always hung out together around the same rock. No fights, no arguments, just hanging out chillin together.I thought firefish gobys did best by themselves... they get highly aggressive when kept with similar or the same species.
I have never mixed different species so I'm not sure about mixing different species of Nemateleotris together but certainly having a couple of the same species is fine.