I picked up a set of boss rims. 18s in the front and 20s for the rear. Clearance is not an issue but I was told extended studs would be needed because of the larger diameter rim. Is this true or will stock studs suffice?

Don't forget rims need to be centered on the hub
Bolting a set of aftermarket rims on without the adaptors that center on the Hub leads to vibrations
The studs are used just to hold the rims on the car and not for centering the wheel.

It may not be as much about the diameter of the wheel as the thickness of the mounting pad area. Especially if they are mag or alum rims.

^^^^this