How does this work with Rampant, ERM and other such mods that affect biters in a major way?

I'll say not great at the moment.

With Rampant, it doesn't break the game, but functionality is reduced. Heavy Waves works by updating unit-group gathering time frequently enough that all groups gather at about the same time. Rampant still seems to mainly use unit-groups to attack, but has other mechanisms of sending attacks that would happen outside this mod's control (meaning you can't count on being safe between waves). I also think Rampant doesn't use the normal biter expansion. The generation of Biter Synchronizers depends on new nests spawning, so if Rampant actually fully deactivates expansion then you won't be able to access the new signals in this mod to track the status of the next big wave.

ERM I only tested a little. They still form waves, but it looks like their groups have a high tendency to attack early. And has the same issue of Biter Synchonizers not attaching to their custom spawners.

Compatibility has been somewhat improved with rampant with the latest update, but I still have some bugs to iron out. You will get waves still, and the biter-combinators will spawn, but there are some errors with the attacking unit count.