you want to stay with tires with the same tread depth (or very close) so they are the same diameter...i ran two different sets of tires for a few months- front set had 1/32" tread and rear had maybe 4/32"- however, the car never acted strange; no issues with the center diff/ no strange noises
i didn't have a choice, but if you can afford it...get 4 tires
this is just my experience with running tires that were not within subaru specs, YMMV