id say the poly would eat everything you have but i have 3 in a 150g with severums,frontosa,plecos etc and there are even baby sevs like 2" in there and my 13" ornate my 10" retro and my 7 inch albino dont bother any of them...my ornate could easily eat everything in there.. but i only feed him pellets...hes also about 2inch thick. but id think cories and small loaches would look like a snack to them. but i think larger cichlids would do good.
and also i couldnt amagine my poly's living happily in a 55g...