You will want to contact the manufacturer of the product to get their recommendations on the type of fasteners, the fastener spacings, and the necessary penetration of the fasteners.
Yes, you will need flashing around the entire perimeter if you wish the roof to be waterproof.
I have to say that I am no fan of cordless framer nailers. I have had two different ones, a Porter Cable and a Pasload. Both were gas fired. The PC was a disaster. Had lots of problems with the propane cells seating properly and nails jamming. Got the Pasload, worked OK, but slow and the smell was so bad that I only used it outdoors. The gas cells are expensive also. After two days on a job, I went back to my trusty Bostich 88 and ran the compressor off a gen set. The other thing I hated was that it could not shoot 10d wire collated nails that I use in my other air framers. Those plastic bits flying at your face sting and you have to reload much more often. Absolutely hate plastic collated nails!
To be honest, I have not tried one of the new battery operated models yet. For framing, I like the largest, thickest nail.(3 1/4") and most cordless ones don't shoot the same larger nails I use in my Bostich.
Since your projects are at home and you already have a 135# compressor, I really think you will get a lot more nailer for the $$$ if you stick with a Bostich 88 mag or similar. You will be able to shoot 4d to 10d nails. The unit itself is much lighter, cheaper and very dependable.
For what you pay for a good cordless, you could own a good framer, 16ga finish and 18ga brad gun, thus having the right tool for every nailing job.
I have to say however, a good cordless finish gun is wicked handy and the battery models have plenty of power, but this humble carpenter is not sold on cordless framers.
Best Answer
First concideration, I would encourage you to use a T&G roof sheathing such as AdvanTec instead of standard CDX plywood. This will give you a better fit and not have to worry about spacing gaps in the plywood. The price is comparable, so there is no cost advantage to using regular plywood.
The sheathing should be secured with 6d galv ring shank nails at apx 8" spacing. 1 5/8" coated drywall screws are great, but more time consuming to install. Don't use non galv common nails, as they tend to back out after a few years.
If your sheathing fits properly on the joists with 3/4" overlay, nailing straight down is fine as long as you bite into solid wood. A slight angle towards the joist is OK as long as the heads of the nails or screws don't protrude above the sheathing. Protruding nails can damage your roofing material, assuming you are not planning on using a layer of Grace Ice and Water shield under your final roofing materials.