Can I use the same segment for different projects at the same time?
Yes, any given segment can be used in multiple projects.
Can I edit a segment that is already associated with a live project? Or remove/swap the segment associated to a live project while the project is live?
Yes, you can, but you will have to be mindful of the potential consequences of your change on the live project. If you are want to make changes for a different experiment you can always clone the segment and make changes to that.
Can I apply more than one segment to my project?
Within a given project (e.g.: Test 1 – Homepage ABn), you can apply multiple segments as long as you have multiple splits (e.g.: Desktop run, Tablet run, Mobile run). But you cannot apply more than one segment to each individual split. For example, if you have two separate ‘Desktop browsers’ and ‘IP address to exclude’ segments, you would not be able to attach them both to a single split-run in your project. But you can easily work around this constraint by creating a new segment which will combine both existing segments (‘Desktop browsers + IP addresses to exclude’), and then attach this.