External compression tools for database backup files Can I use external compression tools like WinZip, 7zip or winrar to compress rman backup files? 

The DBMS is Oracle. A senior DBA claims that backups could be damaged by external compression tools. He argues that the tools shrink blocks which are not full and at a restore the database might have problems because control sums don’t match. I didn't find a source on that.
Since the backup is just a file, you can use whatever file tools there exists, including any and all compression tools.

In order to restore, you will ofcourse need to uncompress the file first.

A far better solution is to use any built-in compression of the backups from whatever sql engine you use. That saves both IO, space and time.