I'm pretty sure we had at least one Slytherin in the DA during DH playtime, so while she would have had to work a bit harder at "proving herself" before she'd have been trusted by your average Gryffindor, I see no issue with you having her as a member of the DA. As for the Order... as she would still have been in school when Voldemort was defeated, and the Order had basically existed to fight Voldie, it really existed in name only after that time, so there wouldn't have been a lot of reason for people to join it - though of course she would have been welcome to join in with the rebuilding of the school and that sort of thing.
Hope that helps a bit!
Carys x