well, first off none of those fish are actually gobies - the mandarins are dragonettes, and the firefish are what they call "wormfish" (just a bit of trivia for ya...)
Basically, unless you have an established tank with lots of LR i'd stay away from the mandarins. they are extremely hard to keep since they live off the copepods in the LR, and very quickly out-eat themselves, and unfortunately i've very rarely heard of them eating anything else.
Firefish on the other hand are very cool fish and relatively easy to take care of. I used to have a red firefish until he vanished (i think this wierd crab that smuggled in on some rock got him
) They do well individually, but look really cool in shoals. 3 of them swimming together look really nice as long as there are enough crevaces in the rock for them all to hide in.
Well, i hope this helps, and i'm sure you'll get more responses... good luck