Can iWatch series 6 pair with iPhone 6 Plus?

No.

You need an iPhone that can run at least iOS 13 or later.

Only with an iPhone 6s or later model.

Those two device that you mentioned will not pair. The Apple Watch Series 6 will come with watchOS 7.x installed. The iPhone 6 Plus is limited to iOS version 12.4.9.

The thing is that the Apple Watch Series 6 will require an iPhone with at least iOS version 13.

Axel F.