There is a possibility that Nika fruit can have any fruit ability, or change own abilities threw "reopearing" in this world. SO that could mean that any fruit can became Nika.
Reread 1037 they make it quite clear that they have been unaware of Nika and this fruit up until recently cause the world government of the past changed the

Then in 1044 they talk about how the world government has never succeeded in getting the gum gum fruit and they talked in the third person.

They are not immortal they’re just the current gorosei
 
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