It sounds like you don't just want to use your mixer for IEM's, you also want to use it for the FOH mix. I'm not saying that won't work, but it probably won't work very well, for a lot of reasons. That's called mixing from stage, and while it's not a new concept, it usually doesn't sound very good.
A band mixing their own monitors on stage is very common though, and can work well. The typical way to do it is with a splitter snake. A simple splitter is basically a multi-channel Y-cable, better ones have ground lifts and maybe transformers. Get yourself a basic splitter, or maybe just a bunch of Y-cables if you want to do it on the cheap, and use your little mixer for your monitors. Plug one output of the splitter into the inputs of your mixer and the other output of the splitter into the FOH snake and let the sound guy do his job.
You could use the main output from your board to mix FOH if the place doesn't have a sound guy, or you don't have a better option for some reason, but giving FOH a split will almost always give you the best results.
