Well my opinion is:
Together the 2 forces could stand against the aliens. Freeman would get easy to the Lambda complex and who knows maybe A. Shephard would be with him on xen against the Nihilanth. With the Xen forces defeated and with the combined power of BM personnel and HECU the Race X would be push back to their world beyond worlds. Becouse of this succes The Black Ops would not try to destroy the facility.
But still the Combine would invade us trough the portal storms right ? Or the scientists in the facility would stop them before the Universal Union could invade us ?
That was the fictious opinion.
My real opinion is: If the HECU would help you and not try to kill you… well Half-Life 1 would be a shitty game without future ! )