Some types of flukes are common in wild caught cichlids, especially africans, The small black spots are the encysted flukes, most are diagenic which means they go through a life cylce that includes at least 2 types of host including fish. Common secondary hosts are some types of snails. Without the secondary host the trematode cannot multiply, this type can last for years but is seldom fatal. The other type only requires the fish and can spread, it is also not often fatal. It is a monogenic fluke. In either case it is difficult to treat but sometimes responds to foods containing meds like Praziquantel or other trematode specific med. Just follow the directions in using the meds. The anti-parasite medicated food by Jungle is one that contains prazi and also contains metro and levasole so it will also deworm the fish and clear up any possible protozoal infections (and hexamita) as well.
The spots will be 2-3 mm across and flat or very slightly raised and just under the skin.
The old school way of treatment was a bit draconian, you immobilized the fish and used the tip of a hypodermic needle to pierce each spot and scratch across the fluke, then used iodine on the wound.
A little salt (1 tbs per 5 gallons) will prevent the transmission of most flukes.
About your stocking level, It is true that 10 different cichlids could live for life in a 55g, but only if they were small ones.
Not all of yours remain small, as adults your oscars will get about as long as most 55g tanks are wide, a 55g would be only minimally big enough for a pair of them alone. A 55g is too small for even a single peacock bass which is also a cichlid, but would be fine for a batch of 10 apistos if arranged right. So you can see that it really depends on the types of cichlids that you keep.
You really should consider getting a larger tank or at least another 55g before to much time goes by.