Unlike a lot of others out there I actually think the Stormcast Eternals are pretty good looking models (though I will probably swap their heads with either Greatswords or Bretonnian Knights). Since your base and formation doesn't really matter in Age of Sigmar I'd like to stick the Eternals onto 25mm square bases to make them usable in 8th Edition as well.

So the elephant in the room for anyone that has purchased the Eternals is of course, do they fit onto 25mm bases with a bit of clever positioning and modeling?

2nd onto rules and fluff. My initial thought is to just use WoC rules for them, and allow them to ally into any order army. A few like the winged dudes will need custom rules, but for the most part regular warriors/chosen/skullreapers/blightkings/etc should work fine. Is this something you would be OK playing against if I make clear what each model type is prior to the game?

For fluff I will keep it quite simple, some-such about how the not-Stormcasts are composed of the spirits of slain Battle-Priests who were just too pissed off to end their fights against Chaos purely because they died. Their spirits continued on fighting in the Warp (hence why nobody has ever seen them), and they have physical bodies from the Wind of Azyr made manifest. The events of the End Times and destruction of the Vortex has enabled them to finally return to the Old World and kick some Chaos-ass.