Unless they are a mated pair, you will most likely have issues adding more Firefish to the existing fish. Here is some information from LiveAquaria.
"In the home aquarium, the Firefish Goby needs multiple safety zones amongst rocky crags or outcroppings into which it can dart if stressed. This member of the Gobiidae family is generally not aggressive towards other fish except those of its own species. However, a mated pair can live peacefully together."