I towed 70K of oversized equipment with my Freightliner!
I'd tow with the Yaris. Toyota doesn't recommend towing of course, as the car is small. Europe allows a 700lb towing capacity. I would imagine that this so called 'towing capacity' would decrease when you include driver (240lbs in my case), wife (115lbs in her case), kid and luggage (100lbs). That'd only leave a few hundred pounds that you can tow SAFELY. Keep in mind a hitch and draw bar also add weight (35-45lbs for a class I hitch).
I wouldn't exceed 60mph towing with the Yaris. After some test drives I'm still waiting for the wife to give me the go ahead to order one.... The first thing I'd do would be to install a hitch. If you have an automatic and plan to make weekend trips towing the trailer, you'd be wise to install an aftermarket transmission fluid cooler. If the dealer asks any questions, just say you plan to drive it through the mountains along the east coast and don't want any over heating trouble (or some other excuse so they won't question the towing and therefore possibly void the warranty).
If you don't plan to tow regularly (say, once a month), it might be best to install and then remove the hitch afterward, or at least before you take it into the dealer should you require any work done under warranty.
Alas, THULE makes some AWESOME small trailers that come in at 221lbs and are a mere 4' wide. The smallest can carry up to 1100lbs (too heavy for Yaris at max capacity!). It has skinny P145 tires for the fuel economy conscience and can hold up to 30 cubic feet. It's also aluminum so it won't rust.
http://www.thuletrailers.com/fw/main...tml?LayoutID=9