I'll aim to put one together in the next month, hopefully featuring the tracked vehicle I've been working on for a while (if someone else who's bought the motors doesn't do it before me!)

For the 14.4V motor yes, it will 'work' but I wouldn't advise it unless you took precautions to avoid overheating and stalling. This is a bit of a generalisation though as there's many variables (2WD? 4WD? Wheel size? Traction levels? Run time?) which will all dictate whether the motors are going to cope at any voltage. I should add that running at above nominal voltage isn't covered in the warranty.

I'd personally recommend an XXL, though not to say others wouldn't work too