Cory's shouldnt be in a tank with large or sharp gravel as it destroys their barbels. Ideally sand, if not then really fine rounded pea gravel. Some of my Albino Cory love sitting in the plants, oddest thing ive ever seen (imagine cory sleeping in plants at the top of the tank)
If you want sand but are worried about gas pockets there is a really simple solution, have Cory cats and Malasian Trumpet snails. problem solved. Also dont make the sand deeper than about 2 - 4 Inches.
Argos playsand £2.50 for 25Kgs (bargain)