The garage-band tag was just created, and there are many that should be tagged with this, is it right to edit them all to add that tag?
Eventually yes a tag should be added to all relative questions. If there are a lot of questions though it's not really the best to change them all at the same time as it will flood the active tab which is what most people use to see and answer new questions. I personally when retagging try not to do more then 3-4 questions at a time.
I know there is a chunk of them on the site, but I'm not sure about the actual amount and any question about garage band should already be tagged software so they shouldn't be too hard to find. Just keep in mind the active tab and feel free to retag questions you find.
Yes, indeed it is. If you can, try to edit anything else in the posts that needs improving as well.