So basically a protein skimmer is a filter for the top of the water, and then you need a regular filter for the rest of the water, right? If you don't have a protein skimmer, what happens? Do you get a film or something on the top? A regular filter isn't sufficient enough.
no, most protein skimmers do not skim the surface of the water, and the ones that do, most of them have an attachment that you would have to purchase for them, such as the skimming boxes for the aqua C remoras or the CPR bak-paks. the "regular filter" is mainly just mechanical filtration, but honestly, it doesnt do much but remove large particles. running carbon in a normal filter bag would be cheaper and jsut as good IMO. if you dont run a protein skimmer, nutrients and waste will build up a lot quicker and you are more likely to have problems with your tank.
you can get rid of the slick on top by skimming the surface with an overflow box to your sump/refugium, a skimmer box if you do a Hand On Back skimmer, or you can point powerheads at the surface. this will mix up the oils so that it is possible for your skimmer (without a skimming box) to pick them up.