So it's a Flowflex plate? I haven't worked on one of these myself, but hopefully someone can chime in with actual experience. I would have expected the holes to be pre-tapped, but not all metal plates are. Normally you would just drive the screws in without tapping (it's only recommended for skis with layer construction, to keep from separating the layers).
I do have a question -- you said there was a gap between the plate and the ski, but the screws you are turning only connect the binding to the plate, correct? Is that gap related to something else, and not dependent on the binding to plate screws?
It's not uncommon to see a gap between RF plates and skis at the back of the plate, depending on how the topsheet curves and ski thickness tapers. Maybe FF plates are the same. I guess a picture would help, so we can see exactly where the gap is and what controls it.
Bottom line, if there is a gap between plate and ski regardless of whether the bindings are attached or not, then it's a different issue and may be normal.