I need to drive 7 servos and have a Pololu Mini Maestro 24. I can't use Arduino directly to drive the Maestro. My questions is this: can I control the Maestro serially (Tx-Rx;Rx-Tx;Gnd-Gnd) using any Arduino microcontroller? Any recommendations on which controller is best if I want to use USB to drive serial commands?
Yes, you can definitely do serial communication through an Arduino to tell something what to do.
If you want to have the Arduino connected to a computer and talking to the computer over serial (which you probably do, for debugging if nothing else), at the same time that it's talking to the Maestro, you might want something that had two serial ports, like the Arduino Mega or something based on the Atmega1284p (no official boards use the 1284, but there are a number of unofficial ones). You could always use software serial, though, so this isn't essential.
I'm not convinced you need the Maestro at all, though...