I haven't had any problems playing mpeg files with it. I'm sure like most players (besides ones like vlc), Light Alloy just uses the codecs already on the computer, but I can't be sure. So, I'm assuming you can RM and QT files because they are already installed. This is true for me when I use MPC.
Light Alloy doesn't really do anything that WMPlayer can't do, it's just that it does it better and is very lightweight. It's playlist features are better, mouse features are customizable, and it has cool OSD features like Zoom Player.
Actually, I probably wouldn't have gotten it if it was $19 like it is, but when it was $10, it was a bargain. Call me cheap.