I use compressor/limiter on certain mics to protect the speakers. This unit will not help with feedback at all, in fact, it could cause more feedback issues if not careful!
If you gate the vocal mic, and crank it up, you'll get no feedback...until somebody talks into it, then the gate opens, and then suddenly you've got feedback. It might possibly limit how loud the feedback is though.
your best bet is either a 31band EQ inserted in the vocal channel, or some sort of feedback destroyer. I haven't ever used one, but I've heard that the sabine units were pretty good, and can be had for a good price on ebay. However, a lot of DJ mixers probably don't have insert points, so you'll have to find one that you can plug a mic directly into (it will have a pre-amp) and then into your board.
I wouldn't put it on the output because then it'll try to detect feedback in your music also, and if you have some sporadic loud parts, it might cut those frequencies down and you'll lose the dynamic range of the music.
If you go the 31band EQ route, then check into a Peavey with the FLS. Before the show, crank up your mic, get some feedback, and watch the EQ. A light will be bright at the feedback frequency, then you can just slide that one down a bit. Do it again and again, getting louder each time till your happy with the output, but I wouldn't cut more than 3 or 4 frequencies or you'll start to lose too much. Again, don't put it on the output, insert into the vocal channel. If you don't have an insert, this won't work for you.