Doing a pre-ride inspection yesterday and noticed a small tear in the front tire so I ordered a new ME-880. Bike has a Dunlop on the rear, do you think it'll be ok to mix? Also are the caliper bolts metric?

 

It'll be fine. As long as the tire is something comparable then you won't even notice. The only time that it would make a difference is if you had a sport bike and were taking it to a track. Tire compound characteristics change over cycles of heat/cooling off so even if you bought a new matching Dunlop, the rubber compound still wouldn't be 100% match to your front.
Like I said, this would only matter if you were dragging a knee and going 150.

 

I don't see a problem either but I always have matched front and rear tires, car or bike. Just my personal preference.
The caliper bolts are metric yes.