I have often raised turkeys and chickens together and you do have to watch for blackhead in them but if they are "free ranging" it is much less of a problem. All the experts tell you it is a big NO NO to raise them together....they do need a higher protein, so be careful as the chicks can sometimes get some deformities from too high a protein when very small. One old-timer in the purebred chicken world told me about it years ago and I'd kinda forgotten all the details.
We use a higher protein feed for the old English Game purebreds as it makes their feathers "harder" which is part of the breeds' requirement.
I have the feed mill put DE in all the chicken feed that we buy by the ton, and add it to the mash for the chicks after they are about 3 weeks old. You figure that we only put 50 lbs per ton, so the chicks get a SMIDGEN in their feed. Also, if you have a box for a dust bath, add some DE to that as it helps to keep the lice and all off them too. Use wood ashes and some lime also.
Turkey poults are sometimes hard to get started, they are dumb and need to be shown where/how to eat. If you raise a few chicks at the same time the turkeys will do as "simon says". Many people start out with a couple of chicks in with them just so they can show them how to eat and drink. That's why when so many start just turkeys, they put bright colored marbles in the waterers so they will go peck on them and learn..."oh this is water and I can drink it".... DUH... but that is why a hen shows them what to do...
I maybe lose one out of 20 so expect to raise most of what you get.
You don't want a broad breasted breed to roost, the breastbone is soft and will get deformed if they are sitting on a roost, especially at a young age.. And they don't have the balance so it is better if they don't.
NH is right, there are only 2 broad breasted types, the white and the bronze. The bronze will grow a little slower than the white. They will take about 4 months to reach the 15 lb size so yeah, you need to get them as late as you can. And you can restrict their feed after they get their first set of feathers. I would try to get them in july if possible. You can always increase their feed if they are not as big as you want as it gets closer. And you can kill them when they reach the target weight and freeze them until Thanksgiving. The meat will taste just as good after being frozen as it will fresh. It is the way they are raised and not whether they are fresh or frozen.... I had 20 that I raised and they DRESSED at 28 to 43 lbs. at 6 months. They free ranged and only had 1 that had a crippled leg but it was that way from hatch.