I'm not sure if they are gobies or just quite often reffered to as Firefish gobies, as well as mandarine gobies... I think that scientifically speaking they are from different families... but I'm not 100%
But back to topic, if you already have the mandarin and are getting sump/fug then cool. You should be fine then.
as for the Firefish they can be agressive towards there own unless they are a mated pair... so it will be hit or miss for you I guess.