Unohana isn't one of the most badass, it seems she IS the most badass around.
And i doubt to be Captain-Commander you only need strenght. You need to be wise and know how to lead too. Shinsui knows what he is doing apparently. But yes, in terms of strenght Shinsui would not come close to be able to use your enemy's ashe's and stuff. But we have no clue about his bankai yet so he might be a lot more powerfull then we think.
I guess because it is from hell or something. Or maybe you need to be drained or be able to resist up to a level or Reiatsu
And your confusing me here! D: Would it mean that the small pink haired Yachiru is named after Unohana? Or is Yachiru the name for a female Kenpachi? >.<Also, did you notice the Yachiru part? HAHAHAH! That little girl is what? Her?!???! WTF! I'm super confused right now.