I tiled an polished porcelain floor years ago ,when first about really ,very nice ,but dangerously slippy,knocked a few back over the years and customer actually gladi told them
When in doubt I use Mapei S2 Quick.
If you've got a customer holding you to being fully responsible for the tiling in a building that is highly likely to move by huge amounts, for the relatively small area, I would pass on the job, and wish them luck finding a mug to do it.
