u can proxy one rax against zerg on some smaller maps and expand behind whilst just pressuring with bunkers etr.. but u need to keep a close eye on depots and minerals etr.
only ever proxy maurder in TvT or TvP if u want to add a hellion or 2 to it. but i dont reccommend the proxy maureder + hellion all in because its very specific as a build counter. The mauruder hellion push is totally different, but if ur proxy mauruders.. well doing it with hellions can very easily go tits up.
proxy 2 rax with marines works in all match ups. very reliable too if its not scouted. Could easily take a bronze to platinum with only proxy 2 rax if u can control well enough.