It's a trial and error type of deal....two exactly alike 709's may prefer different ammunition. Same goes for any manufacturer.
Buy some quality American made target ammo (Winchester, Federal, Remington) in different bullet weights if possible....see what functions best and shoots closest to your point of aim.
As far as defensive ammo....any modern American made jacketed hollowpoint will do the job. Just make sure to run enough through your Slim to ensure proper function.