Hey I have found a cheap set of different size pinion gears and I was wondering if they would all fit the rustler Here they are

Here is the information they give you (plus an image)
Robinson Racing
48 Pitch Odd Pinion Gear Set

The SIX-PACK Starter Set is an economical way to test different gear ratios before buying lightened and hardened premium pinions. There are 6 different precision cut pinions that let you find the optimum ratio for you and your machine. 1/8 inch bore. Six 5-40 set screws included. The odd set includes 15, 17, 19, 21, 23 and 25 tooth pinion gears.

Part Number: 1055