Peter will be faster. Keep in mind that what works on the aluminum version of the Xootr/Swift may not always work on the steel. The bike has slightly different dimensions so tires might not fit on the steel that fit on the aluminum one. Seat post is different size so you can't use Dahon stuff. Lots of these little types of niggles. I've had no problem pulling a trailer but the stress ISN'T on the frame. It attaches to my rear axle and I'd not bother with one that clamps on the rear frame section. The BOB trailer works great for me.
How custom are you wanting to go? Maybe buy a stock Xootr and mod it.