6 nails across in FL, TN may be more lax. Go metal if you can, if installed correctly it will not leak. Make sure screws are snug, if stripped they will back out over time. Decking should be min. 5/8 CDX or 1'X....no OSB..in my opinion....for metal with screws...no nails.
Order and pay for your own material. The days of contractor discounts are about all but gone....maybe contractor markup is more the case. Once you order and pay for you material it's yours and if the contractor is not competent....kick him off and you owe him nothing. Most of the roofers I used in FL....6 to 8 men, will strip and replace the average 3000 sqft roof in one day, baring weather and assuming that the decking and framing and facia...etc...are in good condition. So, at the end of the day, or second day, all you owe him is labor....also assuming he gets his final inspection if you have inspections in your area.
From Cracker to Hillbilly......culture shock!