I'd wait until they have two pairs of leaves. Some of them will already have long roots: I wouldn't hesitate to trim these small leaves and add some hormone powder before putting them in individual pots.
And since you have so many, why not try different methods?
Actually, I've never planted any Zelkova seeds. since I have a big one in my garden (about 10 metres high), I just pick out the chance seedlings I find in the lawn or in pots and the rate of success is about 100%.
Edit:
My son has just paid me a visit to get his instructions about watering (here, the temps will probably be 20-25° for most of the week!), and while we were outside, I took the opportunity to take (lousy) photos.
The first one: a bit early to put in a pot, but could work
Attachment:
zelko-div_150412b.jpg
The second one: can be potted, but I don't have the time right now, so I'll do it later. It's really not a problem to pot these tiny things even when they are in full leaf provided that you keep them in a shaded, wind-free place - at least from my experience in my universe, my garden.
Attachment:
zelko-div_150412a.jpg
For instance, this one stayed in a "tomato pot" for several years, the roots were circling the pot that was twice the hight of the battery. I repotted it on April 6th, cut most of the roots and it seems to be happy Of course it needs to be wired, but when it's in full leaves, no one notices the many faults
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