The only ones of these fish I've kept are guppies and platies, so don't necessarily listen to me!!
Platies in my experience are very peaceful and won't pick on anything, I doubt that it woudl be the platies. If you have more male guppies than females, the males may be fighting amongst themselves for the right to chase the females in circles, and they could be inflicting the damage themselves.
Never heard of x-ray tetras, might be a different common name for something I know by another name. At any rate I've never kept them.
I've never kept rummynoses either, but the LFS stocks them and I've seen them going for bigger fish, especially at feeding time. I read somewhere that they were a good fish to avoid for beginners, but I think that was because of their sensitivity to water quality. (That stuck in my head because the main reason for inclusion on the 'beginners avoid' list was aggression, ie. oscars.)
Rams are cichlids. I've never kept cichlids because I'm a relative beginner and I think they're a bit tricker than the ol' livebearers. As far as I know, they are peaceful.
I'd be inclined to suspect that it would be the guppies themselves or possibly the rams, but I might be accusing the rams because of the 'cichlid' label. It's just that most cichlids are more delicate and more aggressive than a lot of other fish (ie tetras and livebearers.)
