I would be a little careful as the chooks may peck them if they are not seperated in some way or have a large enough space so that they can escape. I once had a neighbour who decided that his kid's rabbit could spend the night in the chook house with the chooks because the rabbit house was wet, the poor thing didn't survive.
Winglet can you please tell us more about free ranging guinea pigs? I have heard about people doing it, some have had great success while others have not. If I can ever get our fox problem under control I would consider doing it in the orchard to help control grass and weeds where I can not slash. The chooks and ducks are doing a good job but there are some things that they will not eat.
He has LOADS of places to escape when he wants to & plenty of shade throughout the garden as well as plenty of water containers (not that he drinks much). He eats the grass down to an even level, and we know exactly where he's been eating recently as there is a very defined line between the short, clipped area that he's looked after & the long bits that we need to mow... he eats whatever grass it happens to be... not as fussy as the chooks who prefer fresh shoots & green, fresh grass rather than dried hay or brown runners from some grass types.
If you could create a fox-proof ring that you could put around the trunk of each tree in your orchard... maybe a half-donut that you could move around - as large as you could make it... a guinea pig or two in there would eat the grass right down for you. Guinea pigs, like rabbits, prefer company... his girlfriend died when they were both in our metal lawn hutch... and he's befriended Harvey, the largest of our male rabbits, and he hangs out with the ducks at times.
If you have a fox problem, or have feral cats around, I wouldn't allow them to free range... they're FAST, but quite a tempting morsel for predators to go after. Not much more I can tell you really... apart from that they're VERY cute, and make an adorable 'squeaking' noise when talking to each other.