Hi,
I started a game with Rampant enabled, and I set the expansion intervals to min = 40 minutes, max = 120 minutes, and I've played for about 6 hours. That should be somewhere between about 3 and 9 expansion events, but when I use the editor to scout the map I see hundreds upon hundreds of bases, covering most of the map. I also saw enemies expand twice within a few minutes of each other. This seems excessive for such slow expansion settings. Is this a bug? Am I misunderstanding something about Rampant and/or vanilla expansion?
Thanks.

yes, you should additionally disable it in mod map settings.
Check off these two:
AI: Enable Migration
AI: Enable Siege AI

There is no vanilla expansion in Rampant since some 0.18.X version. Only its own logics. And it can be very aggressive if you pollute a lot. Unlike vanilla Rampant expansion works as an alternative (instead of attacking your base) reaction to many player's actions - pollution, aggression, expansion close to enemies.
Those two options should disable it. It worked a few years ago. Don't know for now - I never turn them off since the same 0.18.X version, because it becomes way too boring (yes, I miss vanilla expansion logics at such moments too).

This is on the todo list as a bug.

How the vanilla expansion plays with Rampant expansion is something that I haven't settle on yet in terms of how those two mechanics should interact.