Gai and zabuza are the same age (about) so I don't think he was a 7 swordsman at like 10
Yet, when Kishi edo'd the seven swordsmen he had Zabuza in the group with Jinpachi and Mangetsu and the guy Kisame stole his sword from. Basically what I'm getting at is that until this chapter there had only been 1 group introduced as the seven swordsmen of the mist and that included Zabuza, Mangetsu, Jinin, Kushimaru, Jinpachi, Fuguki and Ringo.
Also according to this picture from Naruto Wikia
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Zabuza was part of the latest and strongest generation of swordsmen.
I don't think technically Kisame or Chojurro should count as members of the seven sowrdsmen of the mist, Kisame because he isn't a member of the mist anymore while wielding his blade and also stole it from Kirigakure. Chojurro shouldn't really count because there is no group for him to be a member of, now if Suigetsu achieves his goal of re-instituting the swordsmen and allows Chojurro in then yeah I would say he counted.
As for Zabuza's age remember he did kill what was it 100 other students during the mist's chunin exam and he wasn't even enrolled in the academy at that point, so I wouldn't rule it out of possibility. Also according to his post in Narutopedia
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The exam that Zabuza destroyed was an exam meant to find suitable candidates to join the 7 swordsmen but it says he didn't join until after being an Anbu but that could still only be 2-3 years later.
Anyway my point was where do these 2 new(excluding Raiga) swordsmen fit into the chronology? Were they before Zabuza? They have to be right. Does that mean Kisame stole Fuguki's sword while Zabuza was a swordsman since he was in the group with Fuguki as well?