Its hard to answer your question canucksar because every pheasant is different. There is no right or wrong answer. It would depend on each individual birds personality. This can be influenced by how long you have had the bird, how close it is to being wild genetics, and even what species of pheasant you are talking about. It will depend on whether or not your pen is netted and will you be turning these birds out. If they are flighty, they may kill themselves on th ewire and enclosure every time you go in there. They will also stir up your chickens. Some roosters (chickens) are not tolerant and will pick fights and same goes withthe pheasant. We won't even get into the disease issues. I will assume you have a perfectly healthy flock and ideal conditions, but poultry often carry diseases that effect pheasants.
For the most part, I house and keep mine seperate, even if some do share the same building.
NOW, all the disclaimer stuff out of the way, I have kept pheasants with chickens just fine. I have had outside pheasants meander in with the chickens jsut fine. Its too late for this year, but for future reference, a real good way to make this happen is let a hen hatch a batch of pheasant eggs. She'll raise them and they will act a bit more like chickens. Some pheasants just never calm down though.
I hope this helps.
