I'm using a 22.2v 6s LiPo with 2 BotBitz 85a Esc's. The motors are Rs-550s motors which I checked can handle 24v but maybe I'm just pushing them too hard?
Nah. If they are rated for 24v, the 22.2v LiPo and the 85A speed controller are more than enough. I've ran 4x of the Gimson motors on my middleweight robot Kaizen and that had enough grunt to work happily for a little while but couldn't take the pounding so often ended up blowing one or two after each event. Your arrangement is OK.

Would an all metal gearbox be tough enough to mount straight too then? Cus at the minute the wheels are screwed straight onto the original cordless drill gearbox.
It may be worth checking your gearbox at the moment to see if any of the stages are plastic. An all metal affair will be more reliable when it comes to a pushing contest or flooring it. However, pinning the wheel to the shaft will be the same regardless of the gear material because the shaft will be made of metal.