Well, strictly speaking about 550 size motors: I used a pair of 55t motors in my losi box and they had good torque, but didn't have the RPM to turn the engine over fast enought to get it to start easily...
So, a pair of 27t's might still have the needed torque and also have the RPM, but I wouldn't try anything less (turns) than that. Not enough torque I would imagine...
A pair or 35t or something along those lines might be the ticket.