Will an Apple Watch 5 with Cellular be compatible with an Iphone 6 SE

There is no such thing as an iPhone 6SE. There is an iPhone 6, or an iPhone SE. The Apple Watch 5 will be compatible with an iPhone 6s or above. The cellular activation of a watch would depend on the carrier, but the watch would still pair with the iPhone 6s or above, running iOS 13 or above.

The sequence is iPhone 6s, SE, 7, 8, X, XS, XR, 11 or 11 Pro

So if it is an SE, then it will work with the Apple Watch Series 5

If it is an iPhone 6 or iPhone 6 Plus, it will Not