Fizz is either really, he fits the definition of both set forth by Riot. And I suppose you're correct on the differences between the two roles. I'll fix that up.

I was thinking of touching upon the meta a bit, it's just an oversight of mine that it wasn't in the first draft. It'll definitely be here.

Thanks for the comment, mate.

The main difference between assassins and mages is that mages are ranged and don't always have as much burst (i.e. cass, swain). Fizz is definitely not a mage, he's an assassin.
Also, why don't you explain the meta? That's a big part of lol strategy.

Meta:
Top: Bruiser/tank so he can survive the long walk back
Mid: Mage/assassin so he doesn't have to split experience with support and because flash(-ish)>dash for escaping in mid since it is so small
Bot: ADC and support because the adc needs gold and not as much exp, and to kill dragon.
Jungle: Tank/bruiser/assassin in order to survive early levels in jungle and still hold enemies down long enough or kill them fast enough to help teammates get kills.
This is important so teammates won't rage and so you can counter the meta.

Yup, I am definitely working on that as we speak. Just figured I'd go ahead and publish it now, work on prettying it up after.

Needs BBcode. Nobody will want to read a guide with walls of text.

For BBcoding, read this: Making a guide