You know, you could use the 900's in the mustang, but you might find that they just don't have the power to hold the surfaces under stress (like a hard turn at speed). So even if they work well at slow speed, they might not be a good fit. I know you are into saving money, and believe me, I am too, but you might want to pick up a few more MG14's for the mustang. Either that, or look at the ratings and feedback of the servos in the Mini Servo 11-20 gram category on HK. There are several in the 17 -19 gram weight range that have over 2 kg torque and are fast.
Good luck,
Curtis