Best thing ive used to get them to eat is carnivore pellets. They seem to love those even the ones that are finicky eaters. At that size you will have to break each pellet into 3 probably but its pretty easy to do. Polys, really most fish seem to grow pretty quick on those too.
Also I didnt see it mentioned yet (sorry if someone said it already), make sure you have a lid or some other cover on the tanks you put polys in. They are great jumpers and powerful fish.
As for life in your 120 with the other fish just make sure the cichlids arent beating him up once he goes in there. They are capable of hurting other fish larger then themselves but seldom do. Polys dont really try and fight back to often, and usually end up hurt in "fights" with things like cichlids.