I HC Sombra as Demisexual/romantic because I can’t see her shifting away from her goal (which she seems pretty dead set on) unless someone meant a great deal to her. As for McCree, I can see him either as monogamous or polyamorous, depends on the story, really, and how the characters are written in said story. I think Sombra would be happy to try a one time thing in bed if McCree was really into the idea but other than that, the idea of an emotionally intimate relationship with more than one person might make her anxious (she’s already let one person in, if she lets in more, how safe will they or her second identity be?).
However, if the third person was someone that really cared for both her and McCree and she felt the same way, I don’t think she’d make a decision about it, it’d just be more natural ‘I love you both and I’ll do everything in my power to keep you safe’.
This is all speculative, however, as I still see Sombra as someone who would have a very difficult time developing a positive relationship to just one person, let alone a romantic one. I suppose in this case, I imagine McCree would be a character more likely to be poly than Sombra.
But if you’re just talking pairings, my favorite poly ones with McCree/Sombra would have to be McCree + Sombra + Genji and McCree + Sombra + Widowmaker. 😊