1. I have bitcoins on approx.20 different addresses now. So what I should to do in this case?
You'll have to move/export/import your funds for all these addresses. Note that you should check the addresses you used at the time of the forks, these aren't necessarily the same addresses you're currently using.
2. Never use addresses that was imported? Never use in in Bitcoin and Bitcoin Cash to receiving the money?
Use them to move out your funds in different wallets,
after that, don't use them again for receiving funds (AKA consider them compromised).