Firefish and clown goby would be fine, I have one of each in my 75g. Not sure what Engi goby is, I think you might mean engineer goby. If so, the search I did said they get up to 12" and require a 70g+ tank. I would not suggest them. Here is the site I was looking at if this is what you were considering:
Otherwise, like I said clowns can be real fun. I have a pair of maroons(can get fairly large), but percs or false percs stay smaller are hardy and fun to watch. A tang such as a yellow could be fun too once the tank is established, but would push the limits on size.
It sounds like you have done FW
before. They key to SW
is to do your research. The sight I linked above gives good profiles to help narrow down the search.