Generally, you want to brew the coffee very strong, and you want to add it after fermentation to the bottling bucket - slowly - so that you can add it to the correct taste level.
Others have used coffee in the mash, in the boil, in the primary, everywhere you can think of.
I do not know specifically re: flavored, but I am sure that many flavored coffees would work well! Certain flavors, especially very artificial tasting ones, though, may not work as well but I think that experimentation is the only way to find out!
Good luck with this, I may try a few trial runs of my own!!
You seem to have a like-minded brewer competing for attention, here's the other active post on this:
https://www.homebrewtalk.com/showthread.php?t=53113 .... also, search for "Coffee stout" at the top of the page and you will get ample results. Many good recipes out there.