Re: Vacuum planters?
The seed spacing is changed by a sprocket, same as any other ground driven unit, and by the number of holes in the drum.
The hole sizes and dimple shapes are fixed, but drums with different sizes should be available. If you find one that is close, you could ream/drill out the holes to exactly what you need.
The Cyclo planters had a hard time maintaining seed spacing on more than 8 rows fed from a single drum. The hoses were just too long to maintain a consistent seed travel time. Hence multiple drums for larger machines.
For only 15 acres, several varieties of seeds within that acreage, and less than $8k, I think a Cyclo would be a good fit.
There is a plastic tool that could help you decide. It is a disc that has the dimples and holes in it just like the drums. Part number 113274C3, probably less than $20.