If you can use private keys with no btc on it (because you just sent it to another wallet) will it work with ANY private keys? What if I have very old private keys that had some btc earlier and I'll import it in BCH wallet?
No. That makes no sense at all. The balances on BCH are a "snapshot" from around ~2 PM UTC today. If your address had money on it at that time, then you can import the corresponding key into BCH. However, you should move away the money from the same key on BTC.
The point of that is to avoid giving private keys full of BTC to hosted wallets or largely-untested BCH software.
This. I am not even considering touching anything yet as they made a lot of changes to their software.