For how much coffee to use, check out the recent "Can You Brew It" where they tried to clone Terrapin's Wake-n-Bake stout. They worked from the exact recipe as given to them by the brewer at Terrapin. You can even buy the same blend of coffee they use commercially, if that interests you. I just listened to it this week because I also have a big stout sitting in the primary that I'm going to add coffee & chocolate to. Unfortunately, I don't remember exactly how much they used, because they added ground coffee to the secondary, like you plan to do. I'm going to do it differently.
My plan is to cold brew about a pint of sumatra mandheling using my French press. I'm going to make it a good bit stronger than if I were hot-brewing a cup for myself, probably twice as much coffee as I'd usually use. That would mean two heaping tablespoons (instead of one) per four ounces of water. I'll let it soak in pre-boiled, cooled water in the fridge for about 12 hours, then I'll press it off and pour it into the secondary before racking in the beer.
For my chocolate flavor, I plan to use cacao nibs. I got a bag of cracked ones as a gift. I want to sterilize them, so I plan to soak 8 ounces of them in some Maker's Mark overnight, then add the nibs and the bourbon to the secondary with the coffee before racking. I've never used nibs before, but I've read stuff from people who have and they were happy with the results. 8 ounces seems to be the sweet spot to get good-but-not-overwhelming chocolate flavor. The bourbon will add a bit of flavor, but it shouldn't be overpowering. Last night I did an experiment where I mixed 1/2 tsp of bourbon, 1/4 tsp instant espresso, and 1/4 tsp cocoa powder into 1 oz. of Fuller's London Porter, stirred it well to dissolve everything, then poured in the rest of the bottle. The bourbon was just barely there as a background note. The overall taste was pretty much what I'm looking for in my stout (except, you know, the base porter had less dark malt bitterness and the cocoa powder made it look like a glass of nesquik).