The bacteria in the old bag are dead by now, but the minerals and plant nutrients are still good so you can use the substrate and steal some media from another tank for the bacteria establishment. You will almost certainly get a huge ammonia spike from the substrate so test your levels for at least a week before adding plants.
Using plain substrate or mixing/layering with sand is all down to personal preference and what you want the tank to look like

In my new 20 gal I used fluorite substrate with a sand cap, but it's mixed together quite a bit on its own too.