For the water around here 80 gauge does just fine but if you got huge boulders and rocks I would go with 100 gauge......If you step up to a 115/80 or 90/65 outboard I would go with 100 gauge
The impeller is SS and can take a pretty good beating before you need to take a file to them. Most nicks will come from getting stuck on gravel bar and leaving motor running. Running over shallow bar without dragging at high speed normally want pick up enough rocks to do any damage to impeller.