You should be able to use many different servos provided that they do not exceed 1 ampere of current draw. The vex motor output ports are only rated at 1 ampere continuous draw. So I wouldn't recommend trying to drive some of the monster high inch/oz torque servos that are available, but I'd definitely look at the newer micro and mini servos.
You mentioned other pwm devices. Again if they're some sort of driven device like an actuator, relay, stepper motor with driver circuit, or solenoid, keep the 1 ampere output limit in mind and they must be driven by standard hobby servo pwm signal. If you're looking at devices driven by the digital/analog ports as outputs, sensors using the analog/digital ports as inputs, or devices using the interrupt ports, please check the
specifications here.
Mechanical components can certainly be fabricated or purchased that are compatible with vex. Watch for a post from Artdutra04 soon on some of this info on other commercial parts that will work with vex. Some of these parts FIRSTers are already likely to own, and they're items that have shown up on the vex wish list. That's all I'll say, I don't want to steal his thunder.
