This'll be interesting. I could see them all being 4GB, although the Pro moniker is really looking for some differentiation.
As to the comment about different RAM capacities of the same model based on storage capacity, that's not something Apple has done before, but I know Samsung has done that for the S10+ over in the Android world - sounds very Samscummy. Doubt we'll see that with Apple.
They'll just wait till next year when its time for the 6s to fall off support (so many more years later than it would have if it was an Android phone). But 6s's / SE's are supported for iOS 13.