This is a campaign/rules/style conflict.
My take on the game is that it includes everything we've seen; but that there's a lot of room for expansion. (i.e. Harry hasn't seen everything in the supernatural world...or the real world, for that matter).
One of the players (who's been thinking about being a co-GM) is big on Rules As Written and doesn't want anything in the campaign that's not in the books.
The big sticking point is psychic powers. One player wants to run a psychic with powers not listed in the game book. The possible GM/player is very much against this.
The power in real question is telepathy, a standard psychic (if not seen in the Dresdenverse) power. With proper limitations, I felt that it wouldn't unbalance the game, nor ruin the adventures I had vaguely in mind. He feels very much otherwise.
Primarily, I see the Fate system as allowing a lot of cool things for the players; of me saying Yes as much as possible when a player wants something (and explaining why the game will be better for the group when I say No when that's required). Am I reading something that isn't there?
Your biggest issue is you and your co-GM do not see eye to eye. When I started my game. I said right out I was using the books and as much canon as I can but Albuquerque there is not on the area other than one short story about Ghouls and the Warden Training Facility.
So I said right up front I would be adding in other information that is not in the canon. Harry and his allies do not experience everything in the Dresdenverse in fact reading much of the side notes from Bob and Harry in the books actually mention that. The most famous one is in the Focused Practioner Template Section, where Bob and Harry talk about Chronomancers Your World pg. 77. Harry states he never heard of them. Bob replies that of course you have not.
Now that is not canon because it is not in any novel or short story by Jim Butcher, but the books states they Chronomancers can exist. What about Werebears? Werefoxes? Only weres we have seen are Werewolves up to Small Favor, but there are other shapeshifters just Harry has not met them yet. So your co-GM would ban a player wanting to play a Werefox because they are not in the Canon which is the books and short stories? What if I wanted to play something from Wanted, a Kineticmancer who can control bullets?
The biggest break in canon on this is this: No character in a roleplaying game is canon on a game based on other media such as a book series or a movie. The character created is not canon even if they are a vanilla mortal because they are not part of the general Canon of the story. When you are running a game based on a book or a movie like Dresden Files you are already breaking canon. You are running your own ideas that are not the creators in this case Jim Butcher's. Hell even this FATE system is not Canon it is is just authorized by Jim Butcher with his input.
Telepaths I can see existing just off of what we have read just Harry has not met them. I can see them not favorites of the Laws of Magic and the Wardens might even hunt them down to warn or execute.
That is the biggest part people have a hard time letting go of both GMs and Players that these games do break canon. What is nice is these books are up to Small Favor, those of us that have read Turn Coat and Changes know what is going to happen and can plan accordingly to make it feel like canon but in the end it is not.
I have run RPGs in the Star Wars Universe, the Star Trek Universe, Marvel Universe, DC Universe and the biggest trick is to let people let go of their preconceptions of what they have seen and known.
