yep 10x 10 for a trio is best.
As for mixing, dont, dont care what anyone says, the pheasants will suffer eventually if you give it long enough.
It's not a matter of them fighting or anything, as far as that goes, they will usually get along . But eventually a chicken will get something a pheasant peafowl or turkey cant tollerate.
I know many people do this, but it is not advisable in any way.
Pretty much, if you dont have a pen for the pheasants on their own, dont buy them til you do.
If you have had your chickens for a very long time with no new additions in years, then the risk is lower, but it's still there. Chickens can carry disease's that are deadly to game birds, yet show no signs at all in the chicken, so just be warned on that.