We would never recommend going a step down in a specified thinset, but if the only reason you are using the “Large Format Style “ thinset is to hold the tile in place, then it makes sense that it would work.  You must always be sure the thinset mortar used bonds properly to the substrate, is suitable to bond to the back of the tile, and is appropriate for the type of movement expected.  Back buttering of the tiles is always recommended for larger tile, but not because of our system, it is because the industry recommends back buttering of large tiles in any installation.  Of course the goal is 80-100% coverage of thinset (depends on the tile type and expected use), so however you get there is fine.