well, one there are two threads you should look at both are current discussion threads one on detritus, and the other that is current on refugiums. since you are putting one on your system.
FUGES DETRITUS
rotifiers are copepods, isopods, and amphipods. all similar basically different types and sizes.
and last for tank mates in your current tank clowns, gobies, blennies, and chromis are good types now there may be specific types of these fish that aren't compatible, so research a particular one that you are interested in. Given your current tank size your selection is limited. Tangs, wrasses(which would compete with the mandarin) are pretty mush a no because you don't have enough tank. Oh and becareful some fish like fire gobies are jumpers. heres a compatability chart to help you start.
CHART
P.S. add one fish at a time, and wait at least two weeks before you add another.