Alexander,
I usually only replace the kit fenders with PE ones if I want to add some damage to them, especially some dents and bends along the sides. If I'm only going to bang up the front part, I'll thin the styrene ones and use them. To help keep the PE ones in place, I'll add a strip of plastic (Evergreen is what I use) to the bottom to get a stronger bond or a bead of quick set epoxy will keep them firmly attached.