Correct way to use relay mounting tabs
These are 'Lucar' / 'Faston' tabs, almost certainly 250 series (1/4" wide). These are meant to be used with matching push-on connectors on the ends of flying leads. They are not meant to be plugged into a socket. The relay is meant to be chassis mounted by the mounting tabs.

As for whether the score marks on the mounting tabs can be used to help you break them off, try it. The worst you can do is rip the tab out of the bottom and trash the relay.