They sound really sweet!
"Anyhow some questions is there really a reason not to comingle turkeys and chickens? "
The main reasons they say not to mix them is disease transfer. If your flock is healthy, then it shouldn't be a problem.
"Should Prophylactis for blackhead be provided? Worming?"
If you suspect they could have those things, then treat for them. I've only had a few turkeys and chickens and haven't treated for those things so I can't give you personal experiences on those things.
"What dimensions should a nest box be, and will they use it? (fallback plan is to put the turkey eggs beneath a hen)."
I think the dimensions for the box we made are on my turkey page at
http://www.fishpondinfo.com/turkey.htmOur turkey hen did use the box but it's up to the individual birds. They lay where they want to lay!
"I'm probably over the top but every day I cook some rice or noodles for the birds and they love it, but are these nutritionally sound?"
They probably aren't going to hurt them as long as their main food is still the turkey feed. I don't think it's good to feed them cheese though.
"How high should a roost be? Our local predators are bobcats, coyotes, mountain lion and hawks. So far have only had problems with hawks. Appreciate comments."
With those predators, you should just close them into the coop with roost at night! I don't think you could make one high enough to keep the mountain lions at bay! Hawks hunt during the day and may take poults but probably won't go after adult wild turkeys. Bobcats and coyotes could try to take one off a roost. The roost height should be at least 5-6 feet for adult wild turkeys but since they can fly, it can be even higher. Poults will have to start at lower heights and work their way up. When the foxes got into our chicken coop last year, they got the chickens on the lower 1.5' roost but couldn't reach those on the 6' high roost. I don't know if your turkeys want to roost with their chicken mother and how high she can fly up. We have a pair of Aracauna chickens. I thought those hens don't normally go broody (ours never has).