I use a range of crate sizes (P210) for dogs up to about 40 pounds. All are hard side crates that consist of a top and bottom half and the wire mesh door. All can fit through the door and from experience I have modified them by drilling holes and using nylon wire ties to keep the two halves together. I have had crates fall apart twice when I lifted the crate with an animal in it from the plane so I no longer take a chance.
For larger dogs I use collapsible soft sided crates that I erect once they are in the plane. I have to load them when they are inside the plane and from experience I recommend if those are used to get the crates that can be loaded from the top or side or front. The versatility in loading is sometimes necessary. If the animals get along the larger crates are good for holding multiple animals.
I fly alone on most flights so I do not dare use a harness. I feel it is best to keep the animals contained unless I have a helper, but if flying with a helper a harness is an excellent alternative to crates but is likely to limit the number of animals carried unless you know for sure they are compatible. You do not want to have two animals in harnesses deciding to fight.