Is there a way to target users who are not in another segment? I understand that I could do this via groups, but I'm trying to maintain one group with multiple segments... I'm trying to catch the strange situations that I'm forgetting about in my other segments... Thanks!
You'll be able to create segments based on audience groups, as you've already mentioned, but there won't be a way to create a segment based off of another segment. My recommendation for a workaround would be to copy the segments to their own group, and then you'd be able to set up a new segment based off of the groups you just created as copies of those segments. (This is getting very Dr. Seuss-ish, right? No segments of segments, but segments of groups of segments are ok...)
I realize that may not be a perfect solution, because the group will just be a 'snapshot' of the segment, and won't update as the segment does, but hopefully it may help a bit.
Oh, and if you need help getting those segments copied, you'll just want to use the menu just above the list of email addresses when you open a segment: