I haven't played a BTB campaign, but I have noticed that those who have seem reluctant to share their experiences and opinions. I find this frustrating on occasion.
I can offer some answers based on having used the Blazing Saddles rules since they came out. Flawed though they are.
Armor save--Does the Rhinox have a save on its profile? If so, it receives that armor save. If it does not, it receives no armor save. The ogre is partially protected by the Rhinox, the Rhinox is not protected by the ogre.
Impact hits--sorry no idea.
Ride skill--The Ride skill is taken by the ogre. The Rhinox may not take skills so it can not benifit from them. Do you have a copy of the Blazing Saddles rules? If so, the rules explain the disadvantage of attempting to ride a creature without the skill. If you do not have them I know they are out there on the net if you search, or someone with a link may provide it...