One thing I not sure about is how much current the gearbox motors can take. They have no rating on them and so with the X45 being able to deliver 45amps you could be in a stall situation or uphill etc and burn the motors out. That is why I fused the ESC's with a 25 amp fuse.
This is a common misunderstanding of the way electronics work. Your motors will draw whatever they will draw, regardless the rating of your batteries or ESC. You can offer a thirsty man a lake to drink, but that does not mean he will drown. He will drink as much as he needs and then stop, even though the supply be infinite.
In fact it is important that your battery and ESC be able to provide
more than the maximum your motor will draw, otherwise you risk damaging not the motor but your battery or ESC. Typically as Jurgen says the motor draws the most current when it is completely stalled. If you have an amp-meter you can measure this amount yourself, then you will know exactly where you stand.
Of course it is possible to burn out motors, but when that happens it isn't the current supply that is the cause but rather trying to use an under-powered motor for a heavy job. To use another analogy, if you tried to run a semi truck with a lawn mower engine you would burn out the engine, but it wouldn't have anything to do with the size of the fuel tank.
Your idea of a fuse is still a very good one, although again you'd really want measurements of your system to know what size to use. As Jurgen says, 25 amps may be far beyond the stall current of your motors in which case it will do nothing.
A fuse is a great tool in saving your drive-train during stall conditions. Lot cheaper to replace a fuse than a drive sprocket!