If you are going to haul horses in a converted cargo trailer, PLEASE at least get one that has dual axles. The difference in ride between a single axle trailer and a dual axle trailer is enormous. Single axle trailers have so much front to back tipping--kind of a teeter totter effect, and this does NOT give a good ride for horses, no matter what anyone might claim. It might not be "rough" as such, but it's a very unstable ride for a horse and will be very tiring if you haul even a moderate distance.
The other thing is the floor. I absolutely would not haul horses in any trailer that does not have sufficient cross supports under the floor. I don't know about all cargo trailers, but I know that some of them do not have much in the way of cross supports, they have only the aluminum floor itself. I don't care what you put inside that trailer in the way of wood planks and/or rubber mats, if there isn't sufficient support under that floor then it is not safe for horses.