The gravel and bacteria issue really depends on the size of the tank and whether or not you will cycle it. With an average size betta bowl, it's likely you'll do 100% water changes, or at least very large ones anyway, so it's probably not as big of an issue as if you were going to set up a full tank. That's just my opinion though, I could be wrong.
As for the little bowl - they have to live in those things at the store, so I'm sure a night more will be okay for him. He will be so happy to have a big space to roam around in tomorrow though <img src="/images/graemlins/smile.gif" alt="" /> Just make sure you aclimate him to the water slowly, so he doesn't get stressed out. I hope you find this betta as fun and loving as your first, they really are gorgeous fish!