well...this works for big horses...but I haul in open stock horses all the time, no dividers, just everybody slanted in or side by side (depending on how they fit best) Sometimes I tie sometimes I don't, it just depends on how I'm stacking them in there, and their personalities. If you tie one, tie them all.
IF you do leave them loose (and only if you leave them loose) and they start fussing in the trailer there's a way to get them to stop pretty quickly...I do it to break in my horses to trailer especially if they're 'pushy' or like to act silly like most young horses do the first few times (get a big draft moving too much and watch how much your trailer sways!) You may not feel it as much with a mini (if at all) but if you have a trailer cam you can watch for the right moment or if in doubt just do it anyway. They'll learn quickly to balance and stay standing square.
When they get to kicking or moving too much I tap my brakes, hard! As long as it's SAFE!!!!! (I mean don't do this on a gravel road or in a turn and force yourself to fish tail!) you can pretty much hit them as often as you want, but do it in proportion to the speed you're traveling, please don't go cause yourself to jackknife. But the point is to jar those horses up a bit, make them realize that the important thing is to keep their footing. Silly shenanigans are not put up with in the trailer. It's too dangerous.
Again, only if they're loose, you don't want them to fall when tied (edited to add...BTW they rarely fall when they're not tied, it's when they can't use their neck that they might fall.) It's risky either way because these horses won't know each other and you're confining them to a tight space but if you make them worry about their feet instead of each other it may go more smoothly.
Good luck
