There is NO cli equivalent to auto-add. calibredb add is the cli for the Add books icon on the toolbar, which is a manual operation that does not delete books after adding.
Just put both commands on separate lines, they will be invoked as separate commands.
Code:
/Applications/calibre.app/Contents/console.app/Contents/MacOS/calibredb add --with-library=/Volumes/Ministack/Calibre\ Library --recurse /Users/IS/Dropbox/Reading\ Books/
rm -r /Users/IS/Dropbox/Reading\ Books/*
It will remove all files from the given folder, regardless of whether they are ebooks or whether they have been added to calibre -- although the previous command implies that it has been added.