So your "tire guy" is suggesting you go with both 2 different size tires (staggered), 2 different diameter tires, and 2 different tire brand/models....
Sounds like you need a new "tire guy", Run away fast and do not listen to anything he is saying. All he is doing is trying to push some tires out of inventory or push some tires that he probably gets a small additional bonus for selling. He is putting you at risk to put a few dollars in his pocket.
Staggered is great and all if you like the look, but why the hell would you go with smaller than stock tires on the front, stagger them by getting bigger than stock on the back and leaving the front at the same width or wider. 235's are pretty damn slim in the first place and you are going to have to look REALLY close to see the difference between a 235 and a 225, that is only like 2/3 of an inch.
Bottom line, if you take his advice your going to end up with an unbalanced car, that causes traction control issues, handles weird, and you won't be able to rotate your tires.
I'd say go with a standard setup with a slightly wider tire, 245 or 255(if they fit), and you will be much better off in all aspects.
2012 Mustang GT, Brembo Pkg, Yellow Blaze Metallic