I originally fell in love with coconut curry soup when I worked nights and weekends in my early 20's waiting tables at an Asian BBQ joint. Sometimes, at the end of our shift, the owner would disappear into the kitchen, and she'd return with plates and bowls of food for us to eat. It always looked like she had taken hours to cook the food, but it had literally only taken her minutes.
Up until that point, I had never experienced Thai curries before, so I was surprised by the stark difference between the delicate flavor of Thai curry and the curry flavors that I was used to. If you're concerned about the soup being too spicy (I do recommend using an entire 4 ounce jar of red curry paste, after all), you can reduce the amount. However, if you decide to reduce the amount, you may want to serve some additional paste on the side just in case, and it can be mixed into the hot soup based upon individual tastes. In my experience, Thai curries don't cause that 'burn your face off' type of spicy heat, and the sensation is much more mild than some others I've had.
I didn't add it into the instructions, but you can also opt to cook the curry paste along with the green onions and garlic, which will intensify the flavor a bit. I don't generally do this, because I like the taste of the curry paste as-is. Try it both ways, and see which one you like best!