I'm going to use the screws that came with my kit. They are a little longer (vertically) than the stock Subaru ones but I don't see any problem with them dragging on the ground. Here's a link to the flaps that I bought http://www.rallyarmor.com/store/univ...lap-p-274.html
careful though, I remember that they really ding you on the shipping and handling charges when you buy it from them.
dzemanek4: There's many ways to skin a cat they say. I'll give you a few tips: Use a piece of cardboard cut to the shape of the flaps you buy to decide how you are going to mount them. You could even attach them using removable double sided tape to really give you a visual of how it will look before you start drilling. Make sure that you mount them level to the ground or else it will look weird. If you look at how they mount them in this photo you'll see that you can use the tapered top corner as a guide to make sure that they are in the right position in the wheel well.
Come up with a way to measure the mounting height so each flap is mounted in the same position. I'd measure from the ground to the bottom of the flap (making sure it's level before you drill any holes). Since you don't have any mounting holes you can either drill some using a drill, then get some screws that are just a little bigger than the ones you drilled to attach the flaps or get some self tapping screws from the hardware store or the auto parts store. Self tapping screws don't need a pilot hole to be drilled first, you can just screw them in like any old screw and they should tap right through as long as you get ones rated for sheet metal. They can be tricky though, I've had mixed success with them in the past.
That's all I got, hope it helps.
Couple more things: use washers between the screws and the flaps. This will spread out the pressure exerted by the screw on the flap preventing them from breaking the plastic if you accidentally back them into something like a curb or a rock (which I'm sure you wont).