If you put gravel on top of sand, eventually the gravel will end up on top, and the sand on the bottom, its pretty much inevitable unfortunately. Not sure if the guy at the LFS has ever done this himself, i doubt it, or else he would realise. It would have to be tiny gravel, with almost no gap for sand to filter through
As for the Kribs and Corys mix, im currently looking at info on this right now, as i wanted the same. Its been done many times successfully it seems, but there is a lot to consider, there mustn't be many of them, and there must be a huge floor space to start. im still not sure tho. Its cichlids, some are just gits by nature.
Corys do indeed filter sand out of their gills, they find food in sand like this, as above.
HTH