I don't see Shadow setting boundaries tbh. There is this dialogue combo where Astarion asks, if the others have sweethearts waiting in BG and Shadowheart is like ' eww - relationship. I just seek sex and move on' . She says it more eloquent, but it makes it pretty clear, she is a free spirit. I personally question her taste ( with Halsin, not my very cute halfling lady, she of course is perfect :-D), especially since she has that pretty flirty line with Karlach and no comment on Halsin whatsoever beforehand apart from mocking him.
But generally I think, she is pretty open to a lot of stuff.
And in all honesty, I personally just don't like Halsin and morn the lost opportunity of having a small race companion instead.
