Firstly, welcome aboard! Very cool to see you have an opportunity to lease what seems to be a very wonderful mare :) Too awesome!
As for sneakers, I ride in them. Not often, but if I go to the barn and I decide to ride and all I have is sneakers. I use them. I have been around horses in flip flops, but do my best to limit that because.. Well I like my toes and a horse has a mind of its own. I may have them under my thumb, but one mis-placed bird or something or other would contradict that ;)
As for the shorts, I have no clue how anyone can do that comfortably, but I perspire more on my legs than most... I think.
Anyway, enough of my babble. Welcome!