Seisan Shito Ryu

Yoshukai's version of Seisan is completely different...

that's because they practice the more shuri-te based version, and shito ryu practices the naha-te version. they're as you said, completely different.

Nope, i do gkr... seisan and hangetsu are completely different kata.s
Seisan begins with open hands like sanseru (closed hands) hangetsu beings with an inside hookin block whilst in sanchin dachi

you're right. this seisan comes from the naha-te line, as Mabuni was a student of Higashionna sensei and good friends with Miyagi sensei. however the Uechi ryu version is quite different than this. and this kata was originally named seisan too, but it was a shuri-te version rather than naha-te. Hohan Soken said some did this because they saw similar movements to a kata they practice but didn't have a name for it, so they stole the name in some cases which is why there are many versions possibly.

lol

I praticed this kata with akira sato yesterday...

Good.
Is similar in embusen to Shodokan Goju-Ryu Seisan... Some other Goju-Ryu make 4 steps backwards.

This is not Karate Shyto Ryu of Ishimi :(

It is practiced by Uechi Ryu, Goju Ryu, Okinawan Kempo. Hangetsu is tghe Shotokan version

Shito Ryu has inherited all katas from naha-te and from shuri-te, this explain why Goju Ryu practicings recognize their karate do style.

You may be right. This kata should be in goju because the movements are distinctly goju, but I never had to do this kata while in the goju style. Maybe my instructor thought it was too similar to other katas we were doing. I found that while there were the same katas in shitoryu and gojuryu, the emphasis in breathing techniques in gojuryu to be much different.