Hi Kurt,
Making a copy of the rule(s) is a better way to do it as you can always revert back to the original rule (set it back to active). This allows you to modify/change any parameters, especially when you're doing testing.
As for how MARS prioritize the rule, I am not sure either (doc?), but it probably uses user-defined rules first before moving to other system rules.
User-define rules should be more specific to suit the filtering requirements in specific network environment.
Rgds,
AK