Any question answered...

I wonder if using a metal gear instead would destroy the other plastic gears?

Oddly enough, it's often the other way around. Hard oxide particles stick in the softer material of an unmatched pair and erode the harder material. This is how lapping works. Nylon bushings have been known to wear hardened steel shafts to the point of failure with negligible wear of the bushing. Not that you should worry too much in this case. Nylon or Delrin (acetal) gears are mostly used because they are cheap, not because they are otherwise preferable to metal ones.

Since the jumps between different gear moduli are quite large, you can almost certainly guess what you need simply from counting teeth and measuring OD. The most important thing is finding a stock solution, since having gears made is unlikely to be economically worthwhile.