G.H.
Yes they should pay rent especially when mom & dad are struggling financially, that only makes sense! It's probably easiest to pay one flat amount so that it includes rent, utilities, & food (if they want special food they should pay for that themselves).
I am shocked you are asking if they should do chores. What would be a reason for them NOT to do chores? Unless they want to chip in for a weekly maid service then everyone needs to pitch in. That's 8 people living in one house, that means everyone needs to help out.