I've got a 2-horse side by side, took the divider out, and not only is it more comfortable for horses, as they aren't so confined, and they can ride at a 45degree angle like a slant if they choose. I've hauled a total of 36 different horses and ponies in it, for myself and a rescue I work with, and never had a problem. 31 of these horses had never been loaded or hauled before(under a year of age). 2 tried to jump up in the feed bunk, and soon realized that wasn't a good place to ride.(They were down out of it by the time I was stopped.) I think what the OP was referring to, as far as closing up the back end, is maybe the trailer doesn't have full height doors, as mine, only 1/2 doors, leaving a big hole for a potential escape. (Got 2 panels from Petco, they're wire dog crate divider panels, 36"x36", had a friend weld up a frame for them, and add hinges, and voila, top and bottom doors!) Baling wire would work in a pinch... and a couple of wood pallets. Redneck, yes, but functional!
Tying this colt is your choice... I tied all I hauled except for the 2-2month old babies I rescued back in October. They were definitely more broke for tying by the time we got wherever we're going!
Have fun!