When you're mixing, pretty much the golden rule is just for the mix to be on beat, but you want to try to keep it as original and as entertaining as possible, and you can pretty much mix any two signals together as long as you can make them sound good, but also to keep in mind, just because you get two signals on beat doesn't necessarily mean they compliment each other. So it does have to sound good. You also want to stay away from over-kill with sound, meaning if you're mixing house or you're mixing disco, whatever it is that you're mixing, reggae, whatever genre you're mixing, you want to make sure that the two signals that you're mixing together sound good, because a lot of times, sometimes you have the bass will pretty much fight for the same slot. In those cases, if you're mixing two tracks that are bass heavy, you might want to reduce the low end or cancel out the low end completely one one of the songs, same thing for mids or highs. You can play around with the EQs, you can play around with the volume. Whatever you have in your mind, just try to take different genres and just go for it. Like i said, the main rule is for it to be on beat, and just try to be as original and as creative as you can be.