No, you don't need to pay for an extra portion, unless you want an extra portion. We fed 2 adults, and 3 tweens who were the size of adults off of the regular feast, no extra portions added, and we had food leftover. The only thing to remember is when you go to pick up the food, you need to ask for an extra plate, as it will only come with 4. We've shared a number of meals at The Three Broomsticks and never had an issue with getting extra plates or bowels.
As for the chicken, I can't remember what type it was, but I know there was some type of white meat included as I don't eat dark meat. It was either a combination or they were chicken breasts. Sorry, can't remember exactly, I'm not a big meat eater and paid more attention to the vegetables which I really enjoyed. The kids are little carnivores, so it disappeared fast, and I only paid attention long enough to get a breast. I don't think you get to choose, but it cant hurt to ask politely. All they can do is say no.