There are a TON of threads on this subject, many of which are linked below.
With that being said, its a complex topic that really depends on personal opinion.
There are a few ways to add coffee to a brew
Cold brewed and added to secondary
Cold brewed and added at bottling
Most people tend to use the milder roasts but I actually went the other direction and used a french roast(2.5 oz at flameout) for my RIS that is currently aging.
I guess the questions you need to ask yourself are
How assertive are the other flavors in the beer/how much do you want the coffee to come through
How long do you plan to age this beer
How much do you like coffee
The easiest way to incrementally add coffee is to cold brew and add at bottling. That way you can add a little bit and taste, and keep going until you get where you want to be. You can always add more, but you cant take it out.