Just ordered a brand new caliper as I snapped the bleed nipple off, have ordered a brand new one - will this complicate things having one old caliper and one new one? (e.g the new one being more efficient than the old?)

Naa it'll be fine. I've got one new caliper on the back of mine too, because I had a caliper sticking.

The rear brakes do naff all anyway.

Can't see why it would. Other than the seals, there are no actually wearing parts in a brake caliper, and the pistons hardly move except as a result of pad wear across thousnads of miles.