This was my first year to plant shallots, so I, too, am still learning, but...
Planting in June was probably too late in NE. April might have been better. I am in southcentral KS and planted my first batch during the first half of March. They were purchased from one of the mail order companies and were quite small (3/4"-1" long). I wish I could tell you what variety they are, but can't. Whatever, they have a wonderful flavor! I got a great crop of shallots from that March planting harvested in June (about the middle of the month). I had planted twenty of the "largest" of the small sized sets in March and harvested more than I could ever need for home use.
Having said that, I have taken the SMALLEST sets (which are much larger than the original sets from the mail order company) from the harvested crop to replant this October as an experiment. It IS an experiment as I have gotten greatly conflicting advice as to planting the largest sets or the smallest; the fall or the spring - and I think it depends on the particular strain as well.
So, I must determine for myself whether fall or spring planting is beneficial here. Also, I have heard that planting the smallest sets produces the biggest but fewer shallots; planting the larger sets produces more but smaller shallots?
The point is this - I think that experimentation in your particular clime and soil will tell you the best way to produce them. Try planting your reserved sets from now until the end of September or early October in Nebraska as the temps get cooler and see how they look next year.
I am a hardneck garlic grower, as well. The reverse is true for them (at least here) - while others have told me that hardneck couldn't perform here in KS because it prefers cooler seasons than we have, I plant the LARGEST single garlic cloves in early October and have had phenominal success with the (unidenified) strain that I lucked onto. Huge bulbs. I got the origianal bulbs in Ithaca, NY at a farmer's market.
Alliums are a great study! Good luck...
hortster