I want to keep a betta with some corey’s and i was wondering what would be the best substrate to use, at the LFS they have their Corey's in tanks with sand but I was wondering if the sand would irritate the bettas gills and if i used gravel would this be too hard for the corey's?

I would say that the easier substrate to maintain would be a smaller, rounded gravel. Just be sure that the gravel isn't sharp because that can wear down the cories barbels.