active pups can be mixed with passive pups, no problems in most cases.
an active pup is powered by a battery, thats the difference. usually they use a very, very low output traditional pup with a batterry powered preamp built into the pup.
i used to use emgs and they work fine, especially with high gain. i dont use em anymore cause i think they are somewhat sterile sounding. will you hear the difference between a lp and a strat both with an emg 81 in the bridge? yeah, but not as much as if you had a jb in there. will you hear the difference between an alder strat and an ash one? probably not, while with a pearly gates youd hear quite a bit of difference.
there is nothing wrong with emgs or any active pups, its just not a sound that i need for the tone i want