[i have crabs]

in a 10g if you want the water to stay good enough for the corals limit yourself to mabey 2 small fish,and instead of a powerhead why not get 2 filters to provide flow if you only keep 1 get a mj600 not the 400 i had 2 400's in my old 20g and i felt ripped off for my 60bucks,as far as the skimmer goes if you keeping corals i would get a prysm pro skimmer mabey a used one if possible just because the surface skimmer will keep the top of the water from getting that oily film wich can be a big pain/problem in a small tank.