Very misguieded post
Nothing suggests the Admirals arent as strong as the Yonko - Admiral level isn't a thing - merely 'Top Tier'. Besides, I doubt Xebec was as strong as Newgate and Roger at their absolute peak. I would expect he was around as strong as the current Yonko and Admirals; that's enough to make you legendary, especially with the progeny that he had following his downfall.
According to Garp, nothing like that was ever said.
As for his fruit, that's certainly possible. If I were a betting man I'd say (assuming he's still alive and is the lurking legend) that he doesnt have one. If he isn't the lurking legend and is indeed deceased then we're talking Gomu Gomu no Mi - Roger fought him and defeated him alongside Garp. Is it coincidence then that the fruit ended up being protected by both Shanks (Roger's assumed successor) and Garp in East Blue? Hell nah.
Well i think that Xebec its actually the strongest character in One Piece by the moment. Whitbeard, Shiki and Big Mom were 30/35yrs, we know that the "prime age" is at 40yrs, and Kaido was as strong as Luffy is now or even stronger. Even if he recruited this crew from a game, how could he retain them?
The World Government needed an alliance between Garp and Roger, both at their prime and their respective crews to stop him.
And by the way, we can theorize about a possible betrayel of WhiteBeard, because we know that what WhiteBeard liked was a family, where the partnership and respect were it all, and Xebec doesn't even care about that as the rocks pirates fight each other.
I don't know what could be his fruit the logia of water is a cool ideia, most broken fruit on the manga, countering every other devil fruit user, so only haki users can fight him (Garp, and for the moment Roger, as we dont know if he had a fruit or not). That also helps to understand how he could retain these four legends on the same crew, but Oda said that, that devil fruit doesn't exist. I doubt it was Yami Yami no mi, very obvious move from Oda, also i dont think WhiteBeard would let someone of their crew eat a fruit from someone he didnt like. Maybe he was just the best haki user, that lost to the next two strongest haki users, we dont know, we have to wait to see.