And hey, changelings that can actually attack could make for a fun addition to the swarm.
However, the best part would have to be what mapmakers are gonna do with the little sprites. I imagine entire episodes being reenacted.
Anyway, to answer your questions (and leave stuff for discussion to revive the thread):
1. Bad idea. No good way to implement it. Combining two universes is tough. Putting those two together is ridiculous. That said, I'd be so joyful if it happened.
2. Seeing as it would be a completely separate story, it could happen at any point in the series. Personally, I'd say Brood War so that there's at least the idea that Zerg was invading every planet and found Equestria
3. Wouldn't air on the Hub, but possibly elsewhere. Being a campaign would be more likely though.
4. If nothing else, I wouldn't mind reading a fan-fic on it.