1) Yes, you can mix and match screws and rack and pinion.
2) Everyone building their first machine says speed isn't important, but it is. Really. Especially on a 4x8 machine. Nema 23's are on the small side for a large machine. You want something that will allow you to accelerate very quickly, which needs more power.
3) I think you should seriously consider the Mechmate. www.mechmate.com