If this is the brand you are using below then giving 1 1/4 to 2 1/2 tablespoonsful per gallon of water for 7 days would probably be fine.
These 2 links can help with contact info for testing and necropsy:
http://vmdl.missouri.edu/avian.html
http://www.metzerfarms.com/PoultryLabs.cfm
When I looked it up, it costs $115 in your state. Usually multiple birds are the same cost as one. Getting one tested however is only around $3. Contact your state dept. of agriculture for avian necropsy and testing.
I would recommend getting a necropsy done by your state vet on one if you are going to cull them, just to document it is MG. So sorry about your turkey poults--they look sweet. Where did you get them?
Do you have any place locally such as your county extension agent or even a local vet where you could get one or two tested for mycoplasma? Mycoplasma (MG) is very common in turkeys and chickens. Coryza does not usually affect turkeys, so I would suspect MG, but other bacteria could also be a...