Setting up SDS after a year of non-usage

Hi, I'm setting up a new Sync profile (deleted our old one) and want to create a test Team with using SDS and csv uploads. We do have lots of current teams set up either manually, or via SDS from ages ago. My question is: Do I need to put ALL EXISTING students in the students.csv file and same with the teachers.csv, or can these just contain the users I want to do the testing on?

And I also want to confirm that I won't wreck the current Teams that exist in any way, like it won't remove students from their current Teams and only put them in my testing team?

Thanks.