Sand or substrate especially for plants such as flourite would work well. You wouldn't want anything to to big like gravel because the corys will damage their barbells.
My newest aquarium has laterite as the bottom layer and very fine gravel on the top. I'm excited to see how well the plants do in the finer gravel. Other tanks of mine have laterite on the bottom as well.
Although I've never kept cories, people say they love sand. I personally wouldn't want to deal with that fine of a substrate.
go with sand my cories love to dig up holes and play in sand and i would also consider other fish because your going to be very understocked with just a school of corys
Play sand and pool sand work great. Only problem Ive found with those is that it tends to make the fish colors kinda bland. I prefer black sand from the LFS. Its expensive but it really makes colors stand out.
We use tahitian moon sand (it's black) but it was pretty hard to source in the UK. New corys are shortly to move in from the quarantine tank but our plecos seem to like it and it is nice and heavy so if it gets swished up then it sinks back quickly and doesn't get anywhere near the filter intakes.