Well I don't think tobirama learnt the toad sage mode cause he would have used it when he found out it was obito's weakness which therefore means he didn't go to the toad mountain and which also means he didn't learn that jutsu from the toads,cause there is noway he could have gone there without learning it we should have atleast seen him summon a toad by now atleast,I think that's just a water jutsu I mean common his technique has to do with water pressure and the toad has to do with the strenght and the speed of the tongue unlike minato and the bijudama which is the same except in proportion of chakra,the two just aren't related its like you saying the rasenshuriken is the same as when madara shot a bijudama on a susanoo sword.they just look a like but nothing more,he was a scholar he read and invented jutsus he pribaly saw the toad Mt on a book or something ,oro knew about the snake mountain but he was never there so it is possible for tobirama to know about the toad's without ever being there.
