Yes, this is safe procedure. Just stop using the address after that.
With coinomi you can claim both BCH and BTG ... pay attention to sweep the key to correct wallet.
Which address should I stop using? And why?
Thanks.
The one you will be sweeping. Because after that the private key is compsomised (not so private anymore ) and is much better to not use the address any further.
e.g. there is possible leak due to using hacked device etc.