I would take 10 tails or 9

juubi just because i dont want to choose from the 9

Either shukaku or matatabi

Two tails

Maybe 4 tails or 7 tails

Six tails easily

Is katsuyu a descendant from the Six Tails?

Nothing implies so

Yeah, and aren't bijuus just a ton of chakra with a form? I don't think that'd even be possible

I assumed because they are both slugs that use acid.