if you keep all twelve cories fed enough then they won't compete with the vampire shrimps for bottom food
wafers will be good for both of them incase the cories make a mess of it so the vampires can filter feed
but the vampires will need live plants because they can filter detritus and won't have to worry about the cory competition
I'm planning to do live plants. Just need to get some. That's the hard with all of this (and why my tank is not yet stocked) I live in rural Newfoundland and I am still looking for someone to ship to me.
I found a place but shipping will be $60 :'( I have so make it worth while and get a bunch at once
Vampire shrimp do great in community tanks! Just don't expect to see them for a month or so if you have solid hiding places. I often have to look in every nook and cranny for mine. When he does have a spot and begins to filter feed, you can practice target feeding by dropping food right into the stream he's standing in