It it okay to wear this when horseback riding?

Honestly, unless they're really stretchy the skinny jeans will probably rub and be fairly uncomfortable for mounting and posting. When I was a trail guide people would come in skinny jeans and complain after an hour. Paddock boots with a heel should be fine. I'd say just wear jeans that have a nice flex and stretch to them or khaki's (light color helps with heat). You can get looser jeans at most stores or stores like goodwill for really cheap. Good luck and have fun!