I saw a second-gen Prius last year in that pale green colour, with a "GO VEGAN" vanity plate. Should've sued, since I'm positive I sprained my eyeballs from rolling them so hard.
If you're worried about the synergy of the tire-branding and the livery from another game you've added to the car, er... don't add the livery?
Forza hasn't featured specific brand tires for a while now, they're always "Forza" ones. Outside of NFS — which was, frankly, pretty crap — I can't think of many games that give you multiple options either.
Wheels are handled the same as previous games: you can only get a lower profile tire by going up on the rim size.