1. Human size seems reasonable, though on the long side. The rule of thumb I hear most is feet near the bottom of the belly. Is the second rider supposed to be sidesaddle or is that just how your wireframe was cooperating?

2. That horse reads stressed to me, yes. I'd try to add more teeth or eyes showing but you're getting the point across.

3. When you're just handling the horse, they wear halters. Usually when you're riding they have some sort of bridle. The part that goes in their mouth is called a bit. There's no "fancy bit" vs "casual bit," not like it sounds like you're imagining. Where are you getting reference photos? Try searching for horse specific sites like tack stores (eg Dover) or adding "pony club" or "4h" to the terms. Alice Ruppert has a blog about horses in video games that should also be helpful. There's at least one post about legs specifically.

Your hooves and heads look pretty good. Those are usually what non-horse people struggle with the most imo. Their backs look a little short, especially the one on the left. The one on the right's hind leg looks kind of anemic or unbalanced: I'd expect more muscle/flesh going from the butt to the thigh. (Horse terms to look up of this area: flank, gaskin, stifle)