For a chocolate stout, I would only use the flaked barley and oats, and skip the cara-pils.
Seems like cara-pils is most useful in light-colored and clear beers, but for a stout, haze is acceptable and color's not an issue, so flaked stuff is fine.
Unless you already have otherwise, I'd consider perhaps 1/2 lb each of flaked barley and flaked oats?
If it's still not "full" enough for you, you can always add maltodextrine to your priming sugar mixture at bottling time.