I have diamond gobies and jawfish and some other type of gobies in my 55, no problems at all.
I have seen some of the other gobies get into the jawfish and other gobie's holes when they get frightend, both looking out side by side. Its the funiest thing LOL.
Tank has been cloudy for months now due to them digging and sifting, you need to have super clean sand or yours will be too.You have been warned.
I have a pair of yellowhead jawfish & a pinkspot shrimp goby w/no probs. They even hang out on the same side of tank. (except when the YHJ's spit sand on him)