I had a powder brown (when I first started out and didn't really know better/or didn't really want to listen to good advice
) in a 60g. Basically he constantly tormented all the other fish in the tank, honestly it was because (even though he was only 2") he was a tang in a way to small tank. The powder brown stressed the tank out so much that a bad case of ich broke out. We QT'd all our fish for 4 weeks and left the main tank empty and out of the 6 fish that I had in that tank, only my foxface survived.
Please don't learn the hard way like I did and wait until you get a bigger tank first.
Unless the sleeper goby is eating pellets or mysis I would wait at least 5-6 months (allowing for the sand bed to have time to build up a lot of life in it) before buying one.