A yellow clown goby would work if you are absolutely hell bent on a fish. THat is the only fish i would put in there. PLEASE DO NOT TAKE MMM's advice...that is irresponsible, in my opinion, to have that many fish in a 2.5. Seriously...geez
anyway, NO to the pistol. THey are bull dozers and would need a little more room. WHat i would suggest is a sexy shrimp. They are awesome, entertaining and not too much $$, here is the link:
http://www.liveaquaria.com/product/p...fm?pCatId=1135
I think the sump is a good idea, definately try to make it a fuge too. please dont get more than that clown goby if you get fish!!