Any time the celestial template is applied, it is applied as an inherited template
without exception. It's in the definition of the template. Whatever you are thinking of undoubtedly still treats it as an inherited template, i.e. you summon the creature with that template already applied. You do not call an animal companion, do something with it, and then somehow apply the template. If I'm wrong about this, please point out the feat or whatever that explicitly overrides the inheritance attribute. That's a very big difference, however, so I'm confident that the feat is not worded that way.