I know, while I like Yakuza I yet to finish 0. And I don't want to just buy any game in the series if it could lead to potential spoilers. You don't just want Harry Potter and the Deathly Hallows first and not watch the other Harry Potter movies out of order.
Then again I suppose you could watch Indiana Jones movies out of order given they're mostly stand alone adventures.
It's safe then.
Ishin has no correlation to other Yakuza games other than the look-alike characters in another universe.
Even to Kenzan! (which is set two centuries before Ishin!, it's also different universe, hence the plots are not related with each other)
Kinda like these characters (the difference is our Sonic managed to transport to that universe while in Ishin they still kept separated) :