If you live in an area where Blackhead is a problem, housing turkeys with chickens can be a death sentence to the turkeys.
If Blackhead is not a problem, there are other reasons to not house turkeys with chickens. Turkeys imprint very easily. Turkeys imprinted by chickens lose the ability to understand that chickens are not turkeys. Turkeys being much larger than chickens can be hazardous to the health of the chickens when they treat the chickens the same as they would another turkey.
Turkeys need a lot more personal space than chickens do. You cannot keep them in the same tiny area that you can keep chickens in.