1. The Geckos regulate the current to the drive. The voltage doesn't matter to the motor, but higher voltages allow the current to not fall off at higher speeds. (see 2 below)
2. If the lowest voltage motors you have is 5V, I'd get at least a 50V supply, minimum. Usually 10x to 20x motor voltage is recommended with Gecko's. 30V would just barely get you over 4x on the 7V motors. The higher the voltage of your supply, the higher top speed you're motors will spin at.