the werksan training bumpers are solid rubber with a metal hub - super nice and with a radical matte finish. the only thing i don't like about them is that the inside edge of the outer flange is really sharp and hurts a bit if you slide your fingers over it while loading/unloading. otherwise, these are my favorite bumpers currently.
cheaper bumpers will have more diameter variation (as well as weight), but most are close enough to mix without a problem. i would make sure, however, that your powermax plates are not the largest and taking the bulk of the load--i'm not super confident in their structural integrity. the powermax bumpers we have at mike's gym are reserved for non-weight-dropping activities only. although if you pulled the bolts out and added some loctite to the threads, that will help with the inner part separating, but i'm still not sure the outer rubber will hold up well over time.