You can add dried pepper flakes with no issues and that is the recommended approach. High acid fruit jellies allow for a fair amount of leeway in your own recipes simply because they are so acidic to begin with. So no specific recipe is required.

If fresh peppers are used then it would depend on the amount used. As acidic as most marmalades are given all the citrus ingredients, a small amount of fresh peppers would pose no problem. But I don't know the limit on the amount you could use other than the 1/4 cup that is allowed in most of the hot pepper jellies that are based on 4 cups of juice/ingredients so a 16 to 1 ratio.

Keep in mind the peppers will intensify during storage.

Dave

Thank you, Dave. I will use dried crushed red peppers. I prefer when something has some heat, that I can see the peppers to remind me before using it.