Rim War has a built-in integration with Empire, making the empire settlements a "vassal".

This vassal generates points that are used when defending against Rim War attacks (Scounts, Warbands and Flying Warbands). Empire attacks still happen independently if not disabled.

Important: I personally don't recommend playing Rim War with Empire, vassals generate points slowly, meaning attacks from Rim War are more likely to succeed and cause less profit, happiness and loyalty from your settlements.

The decision is yours, but take into account that using both mods means settlements will be a lot harder to manage.