I would be just as concerned, if not more so, with what the annual frost might do to that rock in terms of shifting. (And the same is true of the posts you've set.) But as long as the foundation can shift without tearing the building apart, you should be good. A lot (most) of the homes built on the permafrost out our way have very heavy beams set of pads. The floor joists are build on these main beams. The beams are stout enough: laminated 2x6 or 2x8 d-fir lumber sandwiched to a thickness of 16 - 24 inches, that they don't flex a lot if one of the pads rises or falls with the seasons and cause great damage to the building. (Doors might not latch properly during some months of the year without shimming.) Too light a 'foundation' beneath the floor might allow a single post or pad to affect the building significantly.
