is Vengeful Shinobi = Ninja ?

Because in wikipedia, the chinese card written it as 惡忍
If my powerlevel doesn't wrong... shouldn't ninja written as (忍)
... or is just nin ?

Also Kunoichi http://yugioh.wikia.com/wiki/Kunoichi which would be translated as Female Ninja ?

(I guess the answer would be the same as multiplication cann't be used on Kuribolt or Winged Kuriboh)

Neither one are Ninja. The Japanese text actually says "ninja" and not "shinobi" on the true Ninja cards.

This is what you want to look for (in Japanese, I don't know anything about Chinese but maybe the kanji is the same):

忍者 (kanji)
にんじゃ (kana)
ni-n-ja

Well this may become a new ruling, with more cards becoming ninjas.

Remember the Archfiends?
A Deal with Dark Ruler
Axe of Despair
B. Skull Dragon
Beast of Talwar
Fiend Skull Dragon
Lesser Fiend
Shadow Tamer
Summoned Skull

Were some of those cards to actually be archfiends, also the cards with "Archfiend" in name, but arnt even archfiend related join their group

Those cards are all considered "Archfiends" because the OCG text contains デーモン (demon) in each of them, and "Demon" in OCG = "Archfiend" in TCG. TCG version just translated all of them differently/badly because the Demon/Archfiend archetype wasn't around when those cards came out so they thought they could get away with censorship.

Konami likes to censor things for the western world, but it's probably just a marketing strategy because they know how many groups would throw a fit if our children were playing with "Demon cards" lol.

Makes sense