Instant/quick/rolled oats are all pretty much equivalent to flaked oats. I sub them pretty regularly. They are all basically pre-cooked and flattened, just to different degrees. But after an hour-long mash in the 150’s, they all work about the same. They just charge more for flaked oats in brewing stores. Just don’t go with something like steel-cut oats, as they are not pre-cooked.
It was just a suggestion, though, there’s absolutely no need to add them. I like to add a small amount to beers like this to help give them a thick, heavy mouthfeel. Doesn’t make it an oatmeal stout, it’s just an ingredient added to provide a certain element to the finished beer. I usually don’t worry about style categories, though. It’s all beer, and it’ll be awesome with or without. 