I am hesitant to put a number on the anthias harem. It really depends on your tank load, total system volume, your husbandry skills, the actual size of the fish, if you're planning to keep them until they are fully grown, how many other fish you want, etc.
You can safely have 1 male and 5 females in the 75 with a few other small peaceful fish. You can probably have more than that, but it's really hard to say without seeing your setup and knowing what the other fish will be.
A 6-fish lyretail anthias harem with a pair of black ocellaris, a coral beauty angel, and maybe a dottyback or a shrimp goby would be a great display.