Welcome to Backyard Chickens, Beth! We are glad you joined the flock! BYC is a helpful site providing all of the information you need to know about poultry. There is always space for members on the BYC roost!
I have never had experience with any Guinea fowl but as long as they aren't aggressive towards the chickens and the amount of nesting boxes is adequate.. then I suspect they should all lay in the nesting boxes you have provided for them.
Welcome to BYC. They may or may not. If free ranged, they will very likely try to hide/steal nests. If in a pen situation, they will lay in the chicken nests, but may also become aggressive during the breeding season.
My guineas didn't like going into the nest box area. They instead would lay in a corner of the coop. Once they were 100% free range they would nest wherever.
How long have you had your guineas? As sourland pointed out the was a noticeable temperament change once they had paired off and mating season started.