Not at all, actually. Halo's can be fired at specific areas, and have their own slip space capabilities. You could very well place all Halo forces outside of the galaxy, including the rings, and fire them at the galaxy. However we aren't actually given a scenario by the OP to say how likely an event like this would occur. If they both spawn in separate galaxies, but are close enough to have meaningful combat, then the scenario is inevitable. They'll commit to that strategy simply because it risks nearly no life of their own.