700c refers to circumference. The other measurement you'd see on tires refers to the width: e.g., 700x23

650s are generally only available on very small road frames. You *shouldn't* have to worry about that based on your inseam. Get 700c wheels if there's an option.

If your tailbone hurts, you're in the wrong saddle. It is probably too big and not ergonomically shaped in the back. Elbow pain is a sign of incorrect fit more than improper form IMO, unless you have some really bad habits. I have known a person to give herself tennis elbow on her road bike, but again, the problem stemmed from her fit mostly.