Yes the 5 Elders rank highest.

Oda was showing this chart at least twice in the manga already and now on page 2 Vegapunk says “No one on the island has a higher command authority status than us”. So he is clearly in the dark about the current situation who else is there. In the last panel, after Atlas gives new orders to the Seraphim, having the angry face of Saturn, makes it clear that he must be stepping on the stage in the next few chapters.

Once the Seraphims don’t follow orders any more, there must be some confusion first. But ‘Apple Gramps’ is quite a smart guy, so I’m sure he will figure it out who else must be nearby or on the island for a big news reveal to the Strawhat crew. Maybe Vegapunk even gives some information he has about the 5 Elders, once they know there must be one of them on Egghead.