Check out the tactica for dealing with cannons: http://www.lustria-online.com/threa...to-fight-cannons-updated-with-responds.13048/ Check out the tactica for ways to build your Cowboy: http://www.lustria-online.com/threads/a-cowboy-for-all.12879/ I think you will find the answers to your questions in these threads.
I use army builder app, and it shows all rules attached to a unit and it doesn't appear in a scar veteran, and I looked through army book and doesn't comment on it in there
Welcome to L-O! Great question and threadromancy. This rule is kind of a layered one. Mounted Saurus become Cavalry models. Models with a unit type of Cavalry are subject to the rules on page 82 of the Big Red Book. One of those rules states that Cavalry have the Swiftstride special rule.
So him rider a carnosaur is riding a monster, Is this classed as cavalry? Because have pay points for his swiftstride ?
Ahh, I suppose I didn't clarify the Carnosaur as that is indeed a mount. A Carnosaur is a Monster and follows the rules for the unit type Monster. A Saurus on his back still leaves it as a Monster. These aren't Cavalry. He does get Thunderstomp though! The Cavalry unit type is intended for things that are equivalent to Horse size, like say a Cold One.