[EdinSumar]

It's pretty hard to say what is hard and fast "illegal" since the legal system of Thay pretty much runs on "whatever the Red Wizards and those few other people in power don't like, yeah, that's illegal". The Red Wizards have banned the worship of Azuth and Mystra precisely because these two gods support the free spread of magical knowledge, and a big deal of the power Red Wizards hold in their own country is precisely because of the monopoly they have. It's an old political thing you see all the time in the real world too, the people with the knowledge holds the power, and the Red Wizards would like to hold on to their power (naturally).

I'd say it very much depends on a ton of factors, but in general (and this is a very big generalisation) most Red Wizards would probably kill off any non-mulan magic user on the off chance that the person is involved in "ilegally spreading magical knowledge". If, on the very very rare off-chance that the person could actually teach them something, they would probably squeeze it out of them first.

Note that this is by no means universal, there are Red Wizards who tolerate, even protect non-mulan spellcasters, for their own reasons, but that is not the general state of affairs.

Sorcerors and Bards, well, innate spellcasters in general are looked down upon in most Faerunian cultures, owing to the fact that they are feared for their power, or that they are the target of jealousy from those who spend years mastering even simple cantrips, or any other factors. A Sorceror or Bard in Thay should probably not be obnoxious about the fact, but since there is lesser chance that they are directly spreading magical knowledge amongst the lower classes, paradoxically, it might be a bit safer to openly be a Sorcerer or a Bard than a Wizard. If they believe you are a Sorceror, anyways.

(A side note, remember that the majority of performers and musicians are not Bards and have no Bard levels. Bards are innate casters who pair their innate magical power (from the same source as Sorcerers, dragon blood, fiend blood, celestial blood, so on) with a proficiency for the arts, lending a focus for it. The majority of performers and musicians are experts or commoners.)

Well, that is my thoughts on the subject, at least.