If this is a reef tank, your stars are not a good choice. If a fish only (assuming you do not have wrasses, triggers, puffers, etc), two would be fine but they must be spot fed.
If this is a reef, you will not get away with 2 Linckia for long...very poor choice for multiple animals as they can not be spot fed and will compete with each other for food (and a 55g is mighty small for most Linckia anyway).
Brittlestars are not a particular problem...again, just keep them individually spot fed.