1. Is it okay to show that I am the boss to other horses [i.e. Making them move their feet if they're in my way, or getting pushy]? The boys were getting pushy as I helped feed today so I moved them out of my space. She didn't seem to mind, and I think respected me more for it. I just want to know for future reference.
It definitely is okay.. I don't allow any horses to push me around, whether it's Indie or a horse when I'm doing barn chores. If they shove me, I shove them ten times harder and usually they're content to just walk beside me.
2. What kind of ground work should I do before riding, so as to enhance my riding and general relationship with the horse?
I usually just do lots of backing up, turns on the hindquarter/forehead, etc. Basically just things you'd do in a showmanship pattern.
3. Should I ride with the owner present at first while I get acquainted with her horses for safety reasons?
I think that'd be up to you. When I leased a horse (although now I own a lovely mare), I had a few lessons on him first before riding on my own (with the two lessons a week as well).
Forever loved, never forgotten; my beautiful Indie. <3 Hoofprints on my heart.