I had a Square Back male with a harem of 3 & they got along in a 125 mixed reef. I would suggest at least this many females, as a minimum ratio. The only aggression I ever noted was the male occasionally chasing the females & all of them darting around at feeding times.
As for feeding, they relish small live foods like brine shrimp, mysis and baby guppies. I weaned the anthias over to prepared foods by mixing the two at feedings. The male eventually began to follow me along the tank whenever I walked by. Something he didn't do for anyone else, except my wife who fed them whenever I was away. They are grazers on the reef, with small throats, and really should be fed more than once per day. Great fish, with lots of personality, in my experience!