All the things I am about to state have been stated in the Anime.

A senin is a Hermit as stated its a ninja which has left his orignal village and no longer works for them basically a rogue who does things on there own. The Legendary Senin's are a group of 3 people that trained under the 3rd hokage 2 of which we have directly meet and the 3rd mentioned by Tenten in what she wasted to be when she grew up.

a Jounin is a elite ninja to be a ANBU member you MUST be a Jounin no exceptions. Pretty much anyone who dies is unranked the gate guards could as easily be Genin's who never advanced as they could be Chunins or jounin's. I get the idea each outpost as shown by the guy who ran the first exam. Has 1 Jounin in charge who gives the orders. While there are several Chuunins stationed all around the city.

Excuse any mispellings or typo's as I don't care to fix them.