I don’t think that’s a practical plan; the combine strategy, as far as we know, is to harvest the resources of captured worlds and use a planet’s own assets against it. They’re draining the oceans and turning humans into soldiers, heavily using existing weapons and infrastructure, etc, etc.
As far as I can tell, getting the combine to expand somewhere and then cutting them off there wouldn’t do much to destabilize their empire, which may very well be why the combine operate as they do. They don’t seem to have moved any unique, irreplaceable assets to earth, even going so far as to appropriate existing leaders, like Breen.
Getting them to expand somewhere and then destroying that expansion would most likely result in a net gain of resources for the combine. That won’t further any goals; it would hardly even be suitable as a method to delay them and work your own plans.
On the other hand, his strategy might be to aid earth just enough to create something - weapons, soldiers, unity between humans and vortigaunts - that can counter the combine, which might be a lot more effective.