If drop springs are used can 17 inch wheels still be put on a 91 mr2?

Yep.

Here's my set up:

Tein S-tech
1.4" drop front
1.7" drop rear

Wheels front 17x7
Wheels back 17x7
Tires front 205/40/17
Tires back 225/45/17

Didn't have to roll fenders, no problems at all.

You'll be fine dropping the car and running 17s :)