A 12.5 tire is way to wide for a 7' rim. I don't think many shops would willing mount that combo together for liability reasons, that width tire on that narrow a rim presents issues with keeping the tire on the bead. In addition to that you would get akward wear on the tire because the contact patch wouldn't be flat at a desirable pressure.I ran 11.5 tires on 7' rims on my Tacoma and you could tell that the rims were almost too narrow.And no you can't mount a R16 tire on a 15' rim. It must be mounted on a 16' rim. R15 tires are for 15' rims, no exceptions in this case. I can't belive you'd even think that a 12.5 tire be to wide for a 7' wheel.
X16 37679 Jerome Id
I have run a mickey thompson 32x12.5 (bias ply) and now a 31x12.5 xterrain (steel radial) on the stock toyota 15x7 wheels. They have held the bead at 5 psi. I have always mounted my own tires and a 12.5 tire mounts just fine on a 7' wheel, the tire does not show any hint of being too wide for the wheel when mounting. Even when aired up the sidewalls don't stress. The only thing is the tire looks smaller than if it was mounted on a 8' or 10' wheel.
X16 37679 Jerome Mo
You also get tons of protection fron hitting the rim of the wheel while aired down.I'm always paranoid about the rims on my fzj80 with the 12.5' tire on the 8' wheels, not enough sidewall sticking out to protect it. I want to go to a 13.5 or 14.5 bias ply (they bulge on the sides, it's a bias ply thing) on there.Here's some pics of my trucks tires.
I belive the xterrain was at 10psi.