Kong had to be one of the Admirals.
Sengoku joined the Marines the same year as Xebec's raid on Mary Geoise, so he can't be one of them.
18 years later, during the God Valley incident, Kong is seen stationed at Marine HQ, giving orders to VA Garp. By then, it seems Kong was the Fleet Admiral, and Sengoku was likely an Admiral.
My theory about the Admiral that Xebec killed is that he was one of the currently unassigned Chinese zodiac signs. If you count Sengoku as the Goat, then only the Rat, Dragon, and Snake remain unassigned.
In particular, I think Oda could make it the Snake zodiac as a nod to our current year, which is the Year of the Snake. I also think it works well since Xebec met Harald on top of the Red Line, and the giants of Elbaph refer to it as the "Serpent of Blood." This was even mentioned just two chapters before Xebec and Harald met.
As for the color, orange is available. It would also fit well if he were Akainu's father, since orange and red are connected. Maybe Akainu had a rough upbringing with a single mother whose husband was killed by Xebec. That could explain the way he was drawn as a child in an SBS.
Additionally, it would fit the darkness vs. sun (fire, magma, etc.) theme: Xebec (father of Teach) killed an Orange Admiral (father of Sakazuki), Blackbeard defeated and captured Ace, and Blackbeard vs. Sun God Luffy is a future fight.
Anyway, Orange Snake = 橙蛇 = Tōja.