have to agree as long as the tire sizes are adhered too then why not, but try to keep the same tread and tire on each axle......I love confusing people with my tire wear story, and here is my story I had the same wear at the rear as you, the garages could not dial out camber and couldnt see any worn suspension componenets and I really didnt want to start changing out parts in the hope they would hit on the problem and resolve it by chance. Was fed up of the rather stiff ride the OEM 19" alloys/wheels gave me, so I picked up a set of 17" OEM (same as the alloy and tire thats used for a spare)...softer ride and better for the crap roads out here......in addition to this benefit there is NO more uneven wear at rear or anywhere for that matter
(PS i also replaced the tires at rear when i had 19" alloys same wear appeared tooooo soon, so the 17" despite looking a bit small are fine)