You can rder the Losi or Associated wheel hexes for use with the XXX-S...the Associated stuff will give you a 2mm wider car... either way they will cost you less than $4.....
I would asume the 12mm (standard) hexes are included in the RTR package... The wheels also may not come pre-glued in which case you can just remove the tires/inserts and put on your X-patterns..... BTW: the losi 8 packs of wheels should be the new standard hex wheels.....