For me, Powers and Templates are both tools. I want to portray a character, and I should be able to describe the character with his aspects and the story behind them. If a power can not be derived from the aspects, and if the aspects just don't fit together, something is off and I would tell the player to go back to the drawing board.
Harry's "Wizard Private Eye" aspect pretty much sums up his powers: he's got the powers of the wizard template, plus stunts to round out his private eye part. The same goes for everything else, if the powers and the aspects don't harmonize, the character is going to feel off.
On the whole, I am not so much concerned about how a specific power is represented mechanically, as long as it fits the characters style. The faith powers for example are all very much different than most other powers, because you have to spend a fate point for most of them to work. But if you take into account the way a true believer should behave, it makes perfect sense. He is (usually) working in a strict code, resulting in all kinds of compels, but in return he can call upon a higher power to aid in his struggle against the darkness.
I actually was surprised that Mouse doesn't have All Creatures are Equal. I think that would have been much more elegant then Sacred Guardian.
On the contrary, it is way more fitting this way. ACAEBG is an outside power, the white god is leveling the playing field for his champions. Sacred Guardian on the other hand is an inherent power of the Temple dogs, because of what they are and where they come from.
Of course any power taken should be within the powerlevels of other powers. Which I think is totally in line for Sacred Guardian. Channelling let's you do pretty much exactly that, only at a much more efficient rate. Sacred Guardian is less efficient, so it costs less. The catch satisfying part might be pretty powerful, but at the end of the day, I don't see it being that big a deal. But I have not had a foo dog in a game, I might be wrong.
Templates represent canon archtypes.
Hmm, I would have to disagree here. To me, the aspects (especially the high concept) represents the archetype. You usually are a "Warden of the White Council" or a "Werecat investigator" or things like that. The main traits of the archetype are defined in this concept. The reason I would not stop there, is that I would also allow things like "Wereparrot Wizard" or "Werewolf by night, Sylph changeling by day". They are not allowed if you strictly keep to the templates in the book, but they could be fun to play, so if a player would come to me and asked to play something like this, I would look at my toolbox (aka the powers list) to see how we may manage to realise such a concept.