An outdoor hutch is entirely natural. Just be careful in cold weather, make sure they have proper shielding from the rain and most importantly make absolutely sure the bottom of the cage is not wire. Wire can cause a very serious and painful issue in guinea pigs called bumblefoot where they get swollen and sore due to the harsh wire on the sensitive pads of the feet. However, it's great that you have more then one as well, seeing as it socialized them. As long as theres no wire flooring, you're doing everything fine!