Installing New Tires w/ Bad Rack & Pinion?

[sd88supra]

I currently have a 225/40/18 front and 265/40/18 rear set up on my daily driver Supra. Because of my financial situation (think broke college student), I have opted to put the tires on from my garaged Supra which has stock wheels with decent Yokohamas (bought them new and stopped driving it a few months later). It's far more expensive to get new tires for my 18" wheels, added to the fact that I need my rack and pinion replaced. I do not want to wait any longer because two tires are very bad.

My question is: what would be the consequences of swapping the tires and driving with the bad rack and pinion for two weeks? I only drive about 60 miles a week, mostly highway, though I do have a 100 mile trip planned for next Saturday, a trip I don't want to do with old worn tires.

Would there be much change in the steering going to a non-staggered, smaller wheel size? I know the constant hitting of bumps with a bad rack and pinion causes uneven tire wear, but I'd be able to get a new rack and pinion and alignment in two weeks.

Thanks for your input.