I'm looking to buy some stock alloy (4 spoke) rims to replace my '94 4X4 stock steel rims. Set I'm going to look at have two that are 225/75/15 & and two are 235/75/15. Seller said they are stock and came off of a 4runner.

Is it normal to have front and rear tire sizes different? Is it ok (mechanically, not cosmetically) to run smaller in front & larger tires in rear ?

I currrently have 31X10.5X15's on my truck. Debating whether to pay to have tires taken off my steel rims and mounted on alloy rims...or....just run the smaller tires that are already mounted on the alloy rims for now.
Thanks for any advice.