Is that true that we have to move all our coins to a bitcoin core wallet so that in case of a fork to Bitcoin ulimited we have our coins also there? I heard that it is not good to leave the BTC at exchanges because they won't deposit these BTU 1:1 to BTC. Anyone knows more?
Depends. I think certain exchanges recently declared they would list both coins, but I don't know how they would handle this. Best is indeed to have the coins, and not exchange IOU.
But don't worry, there won't be a hard fork soon (or ever).
https://blockchain.info/fr/charts/bitcoin-unlimited-shareTechnically, they cannot fork before reaching 51%, but they declared not doing so before reaching 75%.