Yes, you can eat the heritage breeds.
My hens and toms just stopped getting bigger at about 6 months, so I think they do reach an adult size and stop growing. It's the mutant inbreeding of the BBwhites that makes them so unhealthy and ungainly after a certain age.
As for dressing one each month, I think the issue is tenderness. Any heritage turkey before 5-6 months will be nice and tender. Any turkey after that can still be eaten but will need to be cooked longer and at a slower temp to be tender (like a stewing hen versus a roaster). you could probably do one at 4 months, one at 5 months, one at 5.5 months. The toms are bigger than the hens at adult weight.
Other may know more than I on this subject.
Last thanksgiving, heritage turkeys here were selling for $70 to $125 per bird.