I'm not trying to burst any bubbles, but 2.5 gallons is too small for ANY fish and one tiny clown goby max in the 6 gallon. I wouldn't do it personally but if you think you have to have a fish in the 6 gallon, one is plenty.
I have a clown goby in my 16 and he is all over the tank. Think of it this way...just because you can fit in a closet would you want to live in one?