My daughter uses an Apple Watch (1st generation) with an iPhone XS. It works without problems, she just can‘t update past watchOS 4.3.2.
So, in general, you should be able to use any Apple Watch with the iPhone 6. Just make sure there‘s no software version installed on the Apple Watch that‘s more recent than 5.3.7. There might be a more recent software for the AW compatible with devices running iOS 12x. You can update to it as soon as you get the watch.