Does a NULL DATE use as much storage as a value?
If you are using MyISAM, don't.
If you found that DATETIME is 8 bytes, you are using an old version of MySQL; it takes only 5 bytes now.  Upgrade.
&quot;Fixed&quot; is slightly important for MyISAM; rarely important for InnoDB.  So don't worry.  In particular, don't worry unless you have a billion rows.