Sand & Corys.
I never heard or read anywhere that said sand was bad for Corys, the type of sand certainly and this is where common sense should prevail and the sand used should consist of rounded particles, not the sharp or silver sand that is excavated for the building trade. If in doubt look at the grains through a magnifying glass, they should look like tiny pebbles.
In nature probably 80% or more of all Cory species live over a sand substrate. Basically they are filter feeders in that they suck up sand and filter out the food particles, the sand is easily sucked into their mouths without the need to use their barbels for other than what they evolved to be sensing organs. You put Corys on gravel and they physically have to move the particles of gravel to get at the food that is lodged there and this can and does cause barbel wear. The other down fall is that food particles will penetrate deeper than the Corys can reach, were it will decompose and start to foul the the gravel. This in turn means that there is more work for you to do to the keep the gravel clean.
Ian