I used to have a piggy shed and mine lived free range in there.
I layed the entire floor with lino followed by newspaper, woodshavings and hay. I had two open hutches, one at each end filled with hay that they would sleep in. I then provided them with plenty of hides and fleecy beds etc.
My shed had a stable door covered with mesh so that in the summer, we could leave the top half open whilst making sure the piggies were kept safe.
In the winter, I would use snuggle safe heat pads to help keep them warmer as well as adding extra fleeces and hay.
It was predator proofed and we never had a problem with rats and mice. It is important you don't allow this to happen and it must be predator proof even if the guineas are in hutches.
Good luck with whether you have them free range or in hutches.