I would WAIT for the Goby! Let your tank establish. You got a lot in their right now and adding anything else will just make your life more difficult. Trust me! Slow wins this race my friend!
A Goby will be looking for food in the sand, and being you have nothing in your sand yet, it would be a bad Idea to get one. Wait at least 2 months! :-p Than get another fish! Let you system become more established! You can also get Tiger Pods
and start growing a colony of them. Just release them at night when all of your fish are sleeping. :-p