Starting out with a 10 gauge will not be an issue at all as long as your piercer is experienced. Blowing your nose can be a bit awkward for awhile but it's possible. You'll pretty much have to experiment and see what works best for you. As for the nose hair removal you could use a nose hair trimmer as long as you're VERY careful.

They make piercing needles as large as 4 gauge, and you could get pierced & stretched right after.. so 2 gauge is the largest size you Could get pierced at, although it doesn't sound like you want that lol. But like MDME said, starting at 10 gauge shouldn't a problem as long as the piercer has that size jewelry and needles in stock.

I just take my septum piercing out when I have to blow my nose, but I've had it long enough it wont close up or shrink after being out for a few hours. While it was healing I flipped it up when blowing my nose.
As for the hair removal I have no idea lol