Good plan. You can plant the ones for scallions real close,
say about 1.5 inches appart.
Last year I had a lot of them, bough at bargain price and planted them
very close. In the early stages just kept thinning them, using as
green onions. The rest stayed and matured.

Dang it, this website says I did it backwards

http://www.oniongardening.com/planting-onions

Of course I found the site right after I planted the big round sets. I'll hedge my bets and plant the smallest sets in the remaining "big onion" area and see what happens.

stupid onions.

Not to worry. The size difference in a 2lb bag of sets is not that crucial.
How large were the large ones ? As big as a pecan?

The reason they bet on smaller sets is that there is a lesser likelihood
of bolting/going to flower/seed. Obviously, on the extreme, if you plant
an apple size onion, you can bet that it will flower.

But at any event, even relocating onions after they sprout, is quite simple. Just water them real well after relocation.