If I need one teaspoon of dried cayenne pepper, how many will I need for fresh chopped cayenne pepper? Is the powder more concentrated?
Yes, dried spices are almost always more potent than their fresh counterparts. Fresh spicy peppers are particularly tricky because the spice can vary from pepper to pepper. It's difficult to give exact advice without knowing what you're making, but you might consider putting the peppers into your dish whole, tasting regularly, and pulling the peppers out when you get to your desired spice level.
Dried spices are usually 3-4x the intensity of fresh. So make up about 1 Tbsp (3 tsp) chopped fresh cayenne and add it gradually, to taste.