Simple sugars aren't bad but they won't add sweetness. They just ferment out 100% which has the effect of drying out your beer (and boosting abv). Too much and you'll have a thin, "hot" beer. Brown sugar would add some color but you don't really need it since that recipe is already brown.
I've done a lot of coffee beers over the years and I would suggest starting with around 5 cups/5 gal added to the bottling bucket. When I add coffee I brew a pot, dump it in the keg and rack the beer on top. Works great. I've done the French press, cold press, coffee beans in vodka, blah blah blah but nothing works better than simply adding good coffee right before you package the beer.