Is it frowned upon to apply MD/PhD to some schools and MD to others? Furthermore, if I did do that, and I say "MD/PhD" in my PS and activities section would MD only schools just reject me?
I can't answer whether it's frowned upon, although from the MD/PhD's I know all but one aslo applied MD.
With that said, do not put it in the Personal statement if you're applying to both. When you select MD/PhD you have to answer two additional essays, where you are supposed to address why you also want a Ph.D., and which, if you know. You can talk about your love of science/research in your PS, but save your declaration for MD/Ph.D for those additional essays. (Especially if you're going with a gun-ho approach)