Well, I pollarded the volunteertrees for goat fodder today. I can't find in my research where to make the next cuts, but that's a few years down the line. I topped them about shoulder height on me and took the branches down to the second node. I hope I'm doing it right.
Here in Ladakh it''s traditional and common to pollard willows and poplars. All local species of willow and poplar produce abundant shoots after pollarding, but we always do it in late winter / early spring before the leaves are out much. If you pollard in the middle of summer like now, you're likely to take some strength from the tree, but if it's a healthy tree with plenty of water, it'll probably rebound before winter. I hope so!
I don't know anything about pollarding elms, sorry.
Works at a residential alternative high school in the Himalayas SECMOL.org . "Back home" is Cape Cod, E Coast USA.