Is this mod giving biters the ability to merge into a single unit? If yes, how do I disable that?

You can't. This is an important mechanic to prevent lag in the late game.

If its to prevent lag, then why is it causing my game to run in seconds per frame instead of frames per second?
Bugs in my game merged into huge stacked groups with 10-20 biters and each with 700-900. When I try to kill them with light artillery (from another mod) I just remove 1 unit from a stack. When they unstack I have slideshow that looks like factorio. How do I deal with this? The only way I found so far is to just go into the editor, spawn a few nuclear explosions to clear them out.
I vaguely remember someone mentioning that nest have some sort "active biter limit" and they wont spawn more than that. Is that a thing? Are you sure that your stacking works with it correctly? Maybe give us a config to limit that max stack?

"When I try to kill them with light artillery" :
You won't be able to kill a squad with artillery regardless of whether the biters stack or not.
If you destroy at least one biter, the squad will burrow underground and continue the attack.
And if you turn off underground attacks, other methods will work, which I will not reveal. Destroying biters from ultra-long distances is not something I would like to see and will prevent it

Our goals are opposite and you will have to achieve them without my help.

I don't think you understand the problem.
I am using Rampant only for its AI improvements. I wanted to use its biters too, but after I noticed that some mods that add drops to biters ignore Rampant creatures and setting up turret filters takes ages, I decided to ditch the idea entirely, and I opted for a combination of Schall Endgame Evolution and Entrenched Enemies instead. Rampant has underground attacks and all its creatures disabled, as I want only AI changes from it, as normal biters are super dumb, but this leads to the issues mentioned above.
If biters can reach me they don't stack to the numbers that kill game fps, but if they don't it leads fps killing enemy stacking, and that is the core of the issue. Prevent enemies from stacking to the point that it kills performance which is absolutely contrary to what you said in your first message.