Main vanilla problem is laser+flamethrower spam, as soon as you setup your perimeter with these 2 types - you are safe. This mode doesn't solve that problem, because laser still protect you from physical and fire resistant biters and flamethrower still protects you from physical and laser resistant biters so i suggest more complex system:
I ask my friend to make a simple tweak to vanilla biters called "Enhanced behemots" and he teach me how to change values in this mode. I came up with laser resistance (30/80%) and fire resistance (0/75%) What mean after reserach "laser damage 4" upgrade (last "No Hi-Tech Science"), to get 42dmg per shot laser turrets finally start deal minor damage to biters, after fully upgraded they will still have effectiveness below average. Flamethowers will deal something around 36dmg per instance at refined flammables 3 (last "No Hi-Tech Science") and 58 after full upgrade = more than 50sec time to kill (but unfortinatly fire puddles are stacks, so probably 0/75% isn't enough in some cases).
Now i explain my whole mod setup idea. On top of this tweak i had "Explosive biters" and "cannon turret" (just a cannon from tank with same damage and slower rate of fire). This explosive biters is simply a fire immune faction what made flamethrower(overpowered in vanilla) even more useless.
So what i suggest is 3 types of behemots:
Cyan - Laser and Fire resistant, just like from "enhanced behemots" some sort of workhorse for biters (% ammount can be configurable as you already did). They must be beated by only a physical damage, such as machinegun or cannon turrets/explosives (mines, rockets)
Black - Physical, Fire and partially Explosion(50%?) resistant biter. This means his only vulnerability is laser turrets(and explosion part of tank cannon shell), so you cannot just rip laser turrets off because they cannot beat a "workhorse" - most common type of laser resistant biters. Also lasers is to powerful so it would be nice to add even a 0/50% protection against lasers too. I see this type of biters like a rare one 1-5% of population, maybe it's a good idea to give them 2x health pool
Green - Physical and Laser resistant. Their only vulnerability is a fire and you need this flamethowers anyway.
Also there is a way for improvement: add explosive immunity, make them explode after death in short range (1-2tiles) this creates a ultimate breachmaking machine against player's walls and on top of it they should create a poison cloud after death (so they should be immune to poison too) similar to player's throwable poison capsule. You can investigate a "poison" faction from Rampart mod for inspiration about how to code this.
This is a general idea, say what you think and if you give me more time i can calculate all damages and resistances to give you a suggestions not only in types but in numeral (like i did with laser resistance - 30/80)
