Did you look at the link I dug up for you? The HPI MT and the Firestorm series of cars used 12mm hexes and HPI indicate on their website which wheels fit those kits. That's everything you need to know with regards to wheel fitment.
14mm hexes were generally found on the bigger trucks like the T-Maxx and the early Savages. Although at least the Savages now use 17mm or bigger hexes as standard, I'm not sure about the T-Maxx (if they're even still making the 'classic' anymore).
Anthony, he's not refering to the Savage when he says HPI MT; they made a 4wd stadium truck which was named the HPI RS4 MT. It seems that the Megatech Tempest Gladiator is an MT clone. HPI seem to do this after a few years, just sell the designs to a third party and licence them to produce it. The same thing happened with the HPI Baja.