In my experience, chickens and pheasants just don't get along very well. The chickens will pick on the pheasants and vie versa. Also, the feed and space requirements are different for each. Pheasants need a higher protein gamebird feed, while chickens should have a 16% (I believe) layer pellet/crumbles.
Also, pheasants kept for pleasure (that means kept full time with no intent of release) need a very large pen. I've always been told the MINIMUM space for a pair of pheasants is 100 sq ft. That's about a ten by ten size pen containing only pheasants.
Of course, different people have gotten away with doing different things. I tried it once and it did not work for me (In my case, the pheasant I took was a rescue, and I had no where else to put him), but I don't think I would chance it again unless I had a proper set up. Sometimes its better to be cautious than end up with a broken heart when an animal is injured!
Also, i wouldn't recommend keeping pheasants in a cage. When they are younger, that is fine, but once they are older they really need a lot of space to move aorund and fly.
