I searched, but couldn't find this anywhere...

If Cassie is being Team Attacked by a 6 drop and a 1-5 drop, would she be stunned? Her text says she cannot be stunned by a cost of 6 or more, so how would that work? I would assume that she wouldn't be stunned unless the attack of the other drop was enough to get her, but would she still be stunned otherwise?

She's been given errata to make that situation a bit more clear. Her new text is "Cassie Sandsmark cannot be stunned by a character with a cost of 6 or more.
While Cassie Sandsmark is defending against one or more characters with a cost of 6 or more, she cannot be stunned by attackers."

So in your example, she wouldn't be stunned regardless of the 5-drop's attack.