Don't think she is canonically poly, she will be what player chooses.
In my first playthrough, Tav didn't cheat on her, didn't enter relationship with Halsin ( he was always left in camp), didn't visit drow twins, and relationship was 100% monogamous.
If Tav don't cheat on her, she won't either, and she won't mention poly, i think Shadowheart (and Astarion) poly was included only because Halsin is poly (so he can be with somebody else beside Tav).
