No, you won't have any issues with mixing nitrogen with plain air (which is mostly nitrogen anyway).

I have 33x12.5x15 tires mounted on 15x8 rims. I've had them for over three years. I've never popped a bead on any of them, even having aired down to as little as 10 PSI. Got them from Discount Tire. Although I had them mount the tires on the rims and then I just threw them in the bed of my pickup to haul home, they would have installed them on my truck.

When my 33s wear out, I'm thinking I'll move up to 35x12.5x15s.

When I still had the stock rims (15x7), at one point I mounted 31x10.5x15s on them and they were fine.

EDIT: Oh, and I upgraded my brakes to bigger brake disks, but not until about two years after I'd gotten my 33s.