Concerns with different brake types for the front and the rear wheel I'm considering adding a disc fork to my surly cross check and using some avid bb7 road brakes for the front. I've got tektro road levers on drop bars (outfitted with retroshift shifters)  I've got all the gear, so cost is negligible.  The frame is not built for disc brakes, so I would leave my cantilever brakes on the rear.

I know the modulation and grab will be different for the brakes.  Should I be concerned about having different types of brakes on this bike?
Quick answer: Don't worry.

Front and rear are already "functionally different" on two-wheeled vehicles, to the point that we have different tread patterns and even wheel sizes (at least in motorcycles this is common-practice).

So, having brakes with different "feels" is not a problem by itself, unless this bothers you, or makes you over/underestimate breaking power due to a possibly stark difference.

Hope this helps!