I have 1 m and 3 female in a 55g with red zebras, blue cobalts. Only had them for a month and I already have 26 fry from them The male is gorgeous and the females aren't bad looking either. Not aggressive at all. They were actually my second choice as I wanted Pseudotrophues Saulosi but couldn't find a breeding group so I went with the Msobo with no regrets
Yes I have a group and recommend them. They are especially great as a breeding group that doubles as a show tank because both males and females have eye-popping colors. Look for a group with bright orange females. I've been told that males which have been inbred for a few generations (tank raised) tend to lose the contrast in black/blue markings which makes them so distinctive. So if you can get your hands on wild-caught or F1/F2, I'd get that.
I used to have mine in my 125 gallon and moved them to a 55 recently because I got some similar colored Tropheops in the 125. Mine seem ok in the 55, but I've heard of others having trouble with them in a 55 - it may depend on the temperament of your male. I'd shoot for 4 or more females per male.
What else will you be keeping with them in the 55 gallon?
flavus and hongi both have beautiful males.....I think the Hongi females are better looking than the flavus females so I would go with the Hongi over the flavus (if you're looking for a coloful tank), especially in a 55 gal. You could do a group of Msobo, Hongi, and maybe Socolofi or Cobalt. That would be a nice looking tank IMO.