There are some internal differences also, however they are functionally the same. No problem in mixing the calipers.
My belief is that draining the calipers during bleeding is a good idea. Flush fluid from the reservoir to each caliper to ensure fresh fluid is at each caliper. Then drain to get all the old fluid out; otherwise old fluid will remain inside the pistons, particularly on the outboard side. You have to realize that the fluid inlet and bleed nipples are very closely coupled. The fuild will take the shortest path, and that does not include the outer caliper half at all. A simple bleed will never change the fluid in the outer caliper half. The adjuster mechanism in the calipers does an excellent job of trapping old fluid. Only by removing the pistons and rinsing with alcohol can you do a perfect job. Just like stale gas can ruin an engine, so can old fluid ruin a caliper. While this may be critical for racers, a complete job every few years for a street car is adequate.