I've kept Polys with Mega's and Centro's and there was no issue. Neither Mega's or Polys are very food competitive IME so one shouldn't out compete the other. The Centro was more food aggressive then the Polys but not so much that it caused an issue. Dorididae are pretty well protected but still shy away from aggression but I don't see that being a problem with Polys. As for the Mega, yeah he'll out grow the 180 but it will take quite awhile to do so. They're not particularly fast growers.