Is it safe to add other files on the same drive that Time Machine uses as a backup destination?
Some guidelines, Time Machine will not delete files it didn’t write to the Backups.backupdb folder and that folder has permissions to make it almost impossible for someone to accidentally store files there.

Other files outside that directory are as safe as they would be without Time Machine using the drive. (Which means unless they are backed up elsewhere there’s nothing safe keeping one copy of a file)

Time Machine calculates how much space it will need before it starts to copy files, and that calculation is pretty safe in that it over estimates the space it will need (sometimes quite substantially) and it will only delete files it wrote to the protected directory if it needs to free up space.

Once that deletion is done, if you asked Time Machine to notify you after it deletes files, you’ll get a warning that the drive ran low and the backup caused more than the normal deletion to happen.