5/5,5 kg is allready quite a bit smaller than my small patterdale i got right now. stil not near the weight of the praagse ratler though but with a jack russel i at least know that it could be a great ratter with the smaller praagse ratler i also have my doubt. not so much due to size(my way smaller ferrets also handle rats fine) but more because of some described caracter things of the praagse ratler.