I feel that if there are Garlean mages they're either kept isolated and hidden from public view or they all get killed the instant they're found to be able to use magic. Look at how the refugees at that camp treated us and the twins or that old woman we talked to inside the station when we went about getting permission to use the teleportor at the top of Babil.

Though if they were Allagan bio engendered entities then they could be akin to the Ixali who used to have big enough feathers that could allow them the ability to fly.

I'm not confident to say that what happened with Thancred is the same thing as what is happening with the Garleans. As Y'shtola was non corporeal for a lengthier amount of time and is still able to manipulate aether.

Then there is the guy from the fishing studium quests who at the start of the quest line wasn't able to atune to an aetherite due to suffering a huge amount of aether sickness. Only to find that for some reason some hannish alchemists had been working on a gene therapy-esque potion to fix that kind of problem.