Here's a link to a
previous post about driving in this part of Mexico, particularly about driving the highway. If you have more specific questions, ask away.
One oddity that you may find in the Riviera Maya is that the green lights are only for one direction at a time, unlike in the States, where opposing lanes of traffic can go at the same time. This is probably due to the lack of left turn lanes here. Other areas of Mexico have designated turn lanes and/or traffic in opposing directions at the same time.
There are a few other threads on car rental agencies here. I'm too tired right now to dig them up. I've used, and like Executive.