Don't mix substrates, it gets messy, especially with bottom dwelling fishes like Corydoras that dig in the sand. You will end up with the sand mixing in with the substrate. It's a mess. Just use some play sand or small smooth gravel and leave it at that. Sand is better for Cories.
If you want plants, use a liquid aquarium plant fertiliser and don't waste your money on a plant substrate. Plant substrates stop releasing stuff after a period of time (12-24 months) and then they do nothing. Some plant substrates release ammonia during that time and it can affect the fish and screw up the cycling process.