People like Tom do keep turkeys and chickens together, but your housekeeping has to be really good! Worming etc.
We never keep ours together. the turkeys have their own shed and run, which I do use for chickens when the turkeys have been eaten. However, once the chickens are out everything is cleaned top to bottom, washed with Jeyes then Poultryshielded before getting ready for the next lot of turkeys. the runs are washed down with Jeyes, then garden lime and rotovated in. I've never had blackhead in the turkeys.
We got our turkeys poults last Friday 6th August at 1 day old and this particular breed should reach between 6kgs and 10.5kgs oven ready by Christmas. But when we had separate sheds for them years ago, we got them at 4 weeks old in August, September and October so as to get different sizes.