and sound = vibrations. If you can read/hear/understand/predict/whatever something that is transmitted as sound/vibrations, then you're effected by the genjutsu in question. No going around it.
Katsuyu is only as strong as her Summoner. The actual Katsuyu herself is exponentially more badass the more chakra is used to summon her.
That said, if her being a slug means anything, she should be weak against salt. Birds and Toads eat slugs, regardless of their sliminess. Naruto, with his chakra, should be able to out-summon any katsuyu summoner in terms of size. That is the defining factor here. Bigger frogs will simply eat smaller slugs.
