Some models have pre-filters that can be attached, but really their filtering ability is limited. Basicly the pre-filter is mostly used to keep fish from being sucked into the intake. I lost a clown that way a few weeks ago. In other words, powerheads are for water flow.
You want to get at least two. Get smaller ones since your tank is small. As far as placement, it is up to you really, but your goal is to prevent any dead spots and to keep debris from settling so that your filter can from it from the water column. So you want to have a good current going over your rock, maybe bounce one off the front of your glass, point one behind your rock, as long as you get ALL the water moving your good. You goal is to acheive atleast 10x turnover, better than that would be 15x for a fish only system, 20x is you are doing a reef. BTW, I am a huge fan of AquaClear.