I don't think sage mode requires a ton of chakra but the thing is, I doubt there's a slug mountain with slug elders who taught her how to be perfectly still.. that's a frog thing. In Japan, frogs are thought to be mythical creatures with all kinds of stupid crazy powers. I don't know what they think about slugs, but they're not magical.
But pa toad said only naruto can do this training because it requires LOADS of chakra??
I think maybe the slugs themselves have chakra, that may be it, not doing down tsunade, but that would be a LOT of chakra, i mean sakura gets low on chakra when just healing herself a few times against sasori and hitting blocks around, and i know sakuras nowhere near tsunades level, but multiply that by, say, a hundred, thats a big difference, and im sure more than 100 ninjas have been injured.