That sounds like the trailer will have plenty of room. If it is slat sided, make sure the plexiglass is removed. If it's enclosed, make sure every window and vent is open.
I know a lot of cattlemen, my husband included, that drive like he** with a FULL load of cows. Their theory is that the cows are packed in so tight they can lean on each other for support. Probably true, but if one does go down, it is going to be in a world of trouble. If I were you, I would talk to the driver and ask him to be super gentle, hopefully he will understand.
I also have a 3-horse slant, and I wouldn't be too worried about putting 2 odd sized cows in it, especially if they have been kept together. They may ride better together, than being stressed out because they are suddenly alone. When I sold my 3 Dexters, they all rode 3 hours to their new home in my 12'x6' stock trailer - 4 yr old Momma, her 14-mth old heifer and a yearling bull. Everybody did fine.
I wouldn't ride in the compartment with the cattle. Too many chances for you to get hurt. It's also illegal in many states... Could you follow the trailer in your own vehicle? You can easily see the cattle in the trailer if it is a slat sided trailer.
I think they will be just fine. It sounds like you've got a roomy trailer and an experienced driver.