Our trailer is a 5 horse, open stock trailer with a tackroom in the front section...and we've never had even the slightest problem with it as far as our horses are concerned. Of course, all my sister's horses are polo horses, so they're accustomed to being hauled long distances in a trailer like that. Our trailer doesn't have a partition in the middle either, and it hasn't been a problem for us. We have taught the horses to walk in and turn around, and they ride facing the left side of the trailer. It's open at the top, and they can see out with no problem. We put mats on the floor for traction, and if we are going on a long trip, we put shavings down as well. And before you ask, yes, our trailer is of the gooseneck type. We pull it with a diesel truck. Stock trailers come in a wide variety of sizes- you can get one and two horse stock trailers as well as the larger ones- and the smaller trailers can often be pulled with a bumper hitch behind an SUV. One of our former boarders has a trailer of this type, and she uses it to trailer her two horses to lessons and shows. As long as you make sure that the lights and brakes work correctly, then you should have no problem using a stock trailer.
Edit: To Zephania: You need to tell the owners of that mare and new gelding that she needs to be checked for pregnancy. Newly castrated geldings can retain their fertility for up to six weeks after surgery- it takes awhile for the remaining sperm to die. There have been plenty of cases of mares becoming pregnant by newly castrated geldings. I worked in a facility where that actually happened once or twice, so I know what I am saying. And while I was in college, I worked at a summer camp one year where there was a mare who'd gotten pregnant by one of the "geldings" she was pastured with, or so the dealer who brought the horses to the camp told us. The mare foaled on the last day of the season, right before all the horses were scheduled to be picked up and transported south for the winter. Needless to say, I wasn't happy to walk out to the "barn" ( it was really just a shedrow with a few straight stalls in it) from my cabin and find a foal that was desperately trying to nurse and couldn't because the other horses were threatening both him and his mother. Somehow, the baby had gotten himself jammed crossways between the manger and the front rail of the stall his mom was in, and the other horses were biting and sniffing at him. I managed to get him and his mom out of the "barn" and into a nearby paddock, although that was far from simple to do. It wasn't a pleasant experience. When the dealer came to pick the horses up that afternoon, the camp director gave him an earful, because the mare was a horse which should never have been sent to the camp in the first place. NO mare that was that close to foaling ( and I could tell she was getting close during the last month or so of the season) needed to be in a place where she was being ridden for hours every day by a bunch of screaming kids. But since there were only 8 horses in the group to begin with, every horse had to work every day, at least a little bit. Oh, well, you live and learn. My point in telling this story is to make you aware that the same thing could happen in your situation- and unless the owners of this mare and stallion are careful, they may end up with an unwelcome surprise, too, just like I did.
Personal experience. We use the trailer I described in my answer all the time without problems.