PureOS on the Librem 14 will most likely recognize your printer and your mouse without you having to do anything. Have you tried plugging them in?
Also, it is definitely NOT advised to mix in software from another Debian repository or you might end up with what’s called a “FrankenDebian” (a Frankenstein system). I myself was warned about doing this back in 2018 after I got my Librem 15v3.