I've never had a problem transplanting my cloves when they've only just started to show their roots - the birds love scratching around my garlic beds and am always finding random cloves that they've unearthed. so in answer to your question, anytime should be fine. Too small has not been a problem for me although have never tried transplanting when they've got established leaf growth.

I think this works fine for regular bulbils. But for the tiny porcelain bulbils, it may be better just to leave them. They are just tiny.

Too many volunteers is a problem. If you grow more than one variety, then it is very hard to ID them...