Being a cousin clan doesn't mean they are lesser.. they clearly have traits that even the senju clan does not have... aka regenerative properties etc..
Nagato, had enough chakra to distribute to 6 different bodies, fight numerous ninjas in konoha for a few hours, go on to fight naruto, then fight a tailed beast,, lets not forget the large scale chakra taxing jutsu he used like chou shinra tensei, Chibaku tensei, Rinne tensei...
And just as Kakashi gets his chakra drained faster for not being a heir to the eye, Nagato's eye was implanted , meaning everything he did, was much more taxing on him that it would be for a true heir, i,e Madara
so yes, Nagato's chakra levels does rival that of tsunade's , as for Hashirama, we can't possibly know what his stamina was like