You can run different tires. I have done it on sport / tour / motocross for years.
I was on a tour about 2 weeks ago with about 20+ other VFR riders. Some of the combo's they ran:
rear / Frt
BT21 / Metz M3
BT21 / Mich PP
MichPR / Mich PP
A lot of rear tires where based on how they rode ( race/tour )and weather conditions. The Mich Power Pilot front seemed to be the choice of most. Depending on who you talk to, some get great milage, some dont. Question is, "how do they ride". I know one guy that hit 8,000km on his BT021 rear. He lives in the Kelowna area with twisty roads in his back yard.
My BT021's are currently at 11,000 KM.... .... Welcome to Alberta. sigh.
I will be going with the BT21 rear / Mich Power Pilot ( front ) combo when ready.
The rear BT021 is great in the rain storms I have been hit with ( 2weeks ago -Revelsotke to Airdrie) and twisties. The 21 front is wearing a little to fast.
My $0.02 worth.
(added)
** check a few dealers in Calgary and edm. The BT021 are being replaced by the BT023 and some dealers are blowing them out. **