yeah get like a 10g and throw some guppies in there, before you know it you will have more feeders than you can handle, but yeah a diet of only goldfish/rosy reds is very unhealthy and fatty, these fish have very little nutrition, the analogy to Mcdonalds is a very good one, besides the fact of there being a high risk of parasitic infection it is just extremely unhealthy and will lead to either the development of HITH or a very shortened lifespan. every once in awhile if they have been throughly quarantined (for 2 weeks at least, but this is very annoying which is why i dont bother feeding these feeders) then it should be ok as a very minimal treat, but he may like them better but its like a child with candy, of course the child is going to choose the candy over actual food but its your job as the parent to make sure they eat healthy. like others have said there are other great options that your O will love just as much, earthworms are always a favorite along with pretty much any insect they can get their mouths on (i usually catch grasshoppers now and then when im outside in the summer and chuck them in there) but easiest way to give full diet is to get him on a staple of pellets and then just give treats of the other foods
after all that in short, to answer your question, as long as your tank and filttration are large enough to handle the bioload of an oscar and very messy goldfish then no, its not bad to have the extra fish in the tank but if your water quality is suffering because of it then yes it is bad, how big is your tank by the way?
IVE ALWAYS RAISED MY OWN FEEDERS CUZ U CANT TRUST THE LFS FEEDERS.
