Hi Deansmig,
How big is the pen you have had built? It is often recommended that you allow a minimum of 2 sq. metres of run floor space per bird, so in your case, the run should be at least 8 sq. metres, and more Is better. Chickens react badly to overcrowding, they can get stressed and fight and pluck each other's feathers, and those lower in the pecking order can find it hard to escape a bully in a run which is too small. A large run is a blessing, it will not get so muddy and dirty very fast, and if is really huge in proportion to the number of birds you intend to keep, you could always divide it and rest one half whilst they are using the other. If you can measure your lovely big run and let us know how big it actually is, people might be able to advise you better.
I wouldn't recommend mixing them with other animals in the same pen. Their needs for housing and feeding are different from other creatures, and if you added other animals or birds it might be difficult to prevent the chickens from bullying them, or in the case of small fluffies, they might get dirty from walking around in chicken poo. If the run is really huge, maybe you could subdivide it and keep whatever else you wanted in their own area?