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G'day Martin,
I'm no expert, but I once studied under someone who
was. With your potential rootstock, what you want is to force as much growth
up through the main stem as you can before they go dormant, so I would be
wary of any pruning above where you hope to make the grafts except to keep
them to a single trunk. You may want to clear growth up to the desired graft
height, but not so as to slow them down. Feed them up with nitrogen and
don't worry to much about shape as you will probably be pruning them back
hard to just a few inches high when you are ready to graft. This chap had
years of experience and I've seen him graft thin little seedlings only a few
months old. Once he demonstrated grafting with a Camellia seedling that only
had just sent up its first 2 seedling leaves, then he cut of its little tap
root, grafted and struck it in the same pot. Amazing stuff, it still had the
seed case attached. So you don't really need much diameter, just practice.
He recommended cold treatment for cool climate maples, about 3 months in the
fridge if I recall. So I would go ahead with next spring in mind. Good luck
and let us know how you go. What are you going to do with all those plants?

China
Wingham
NSW