I know I would feel a little hesitant to put them in but you might be ok with the tank size. I have a pink spotted goby and it's one of the most aggressive gobies I've ever had... Doesn't really bother my other fish unless they get near it's holes...
The Pink Spot is about 5" and the Diamond is 2-3", I'm hoping the Diamond just picks a different corner. I'm gonna take a chance here and hope for the best.
Just to let everyone know both goby's are doing great. The Diamond picked the polar opposite side of the tank to setup shop. Now both sides of the tank look like a bomb went off
Stanski: unfortunately one of the fire fish jumped out of the tank. The other one is doing fine. Never had a jumper. I was surprised. I just bought some plexi and cut it to fit the tank. Hopefully this never happens again!