How viable are all mage teams?

You can do it easily especially if you use different colors. For combos you can just pick your favorites.

Colorless mage unit when.

Full mage team can work, but i don’t advice full blade tome. Usually one is enough due to the offensive power, two if you really want.
If i were you i would build at least 1 close counter mage with decent tanking and an owl tome to have someone that can be played on ennemy phase

Definitely recommend using Ophelia on this team since she'll get so much special charge from all the other mages you could probably use Aether turn 1

It’s gonna be really fun, but no.
There have recently been a lot more units that’s are both good in def and res (such as Fae/Halloween Kagero) that deal physical damage and would destroy most of these mages. The Mages who DO have good defense honestly are not that good now, duh as Robin and Boey due to their horrible offensive stats. (Sorry)

Vietnam flashbacks, when I was using my full mage team and got destroyed by one singular Roy...

I still have nightmares from it...