27 inch or 700c

as long as your brakes are adjustable so they can reach down to the rims of 700c wheels it'll be fine, although i'd probably recommend changing both wheels to 700c..

This, along with aiden's, post are the only sensible things here.
However, aesthetically, a frame built for 27" wheels is never going to look right wlith 700c's

Teenslain's post also makes sense