I can't help with you the printable kinds as when I do full color it is usually embroidery.
But for your items such as cut vinyl, I know you'll need to use the "heat sensitive" types. Leather (like Nylon etc) don't tend to like the higher Heat/Time/Duration settings that you have for your regular shirt vinyl. I tend to like Thermoflex, but I know quite a few people that like Siser for their heat transfer vinyl.
I do think Siser's vinyl is already low enough on the temps that they can be put on leather, but you might want to confirm that. I know with Thermoflex you have to get a specialty one for your nylons and leathers. I would also do two pressings per application as that does give a better "hold" on to the substrate then just one. Especially when you are talking about something that is porous like leather.