Svirfneblin already get 20% resistance to fire, cold, lightning, acid, and energy, and 33% resistance to poison and disease.

Svirfneblin
sc damage
Bash______: Vulnerable____________-33%__Pierce____: Normal
Slash_____: Normal______________________Fire______: Moderately resistant___20%
Cold______: Moderately resistant_____20%__Lightning__: Moderately resistant____20%
Acid______: Moderately resistant_____20%__Poison____: Resistant_____________33%
Negative__: Normal______________________Holy______: Normal
Energy____: Moderately resistant_____20%__Mental____: Normal
Disease___: Resistant______________33%__Drowning__: Normal
Light_____: Normal______________________Radiation_: Normal


Gestalt metamorphosis carries over the 20% to fire, cold, lightning, and acid (and drops the 20% energy), PLUS the 33% poison and disease to their form when they shift.

Diamondback gets -33% cold, and +33% fire, PLUS 90% poison.

(Diamondback BEFORE gestalt metamorphosis)
sc damage
Bash______: Normal______________________Pierce____: Normal
Slash_____: Normal______________________Fire______: Resistant______________33%
Cold______: Vulnerable____________-33%___Lightning__: Normal
Acid______: Normal______________________Poison____: Nearly immune__________90%
Negative__: Normal______________________Holy______: Normal
Energy____: Normal______________________Mental____: Normal
Disease___: Normal______________________Drowning__: Normal
Light_____: Normal______________________Radiation__: Normal

The carry over from gestalt metamorphosis is suppose to stack with whatever resistances the form has, as demonstrated by the diminished vuln cold while in diamondback form, dropping from -33% to -13% after the svirf gives a 20% boost.

(Diamondback AFTER gestalt metamorphosis)
sc damage
Bash______: Normal_____________________Pierce____: Normal
Slash_____: Normal______________________Fire______: Resistant______________33%
Cold______: Slightly vulnerable______-13%___Lightning_: Moderately resistant______20%
Acid______: Moderately resistant_____20%___Poison____: Nearly immune_________90%
Negative__: Normal______________________Holy______: Normal
Energy____: Normal______________________Mental____: Normal
Disease___: Resistant______________33%__Drowning__: Normal
Light_____: Normal______________________Radiation__: Normal

However, notice that the fire resist and poison resist aren't stacking properly anymore. It's been a while since I played a svirf diamondback, but I'm quite certain I remember fire resistance going up to 50% in diamdonback form after the 20% boost from svirfneblin gestalt (with diminished returns putting it at 50% rather than 53%). I forget exactly what the diminished returns does to the diamondback's 90% after the svirf 33%, but for some reason the number 95% is sticking in my head.

I've also had a svirf shifter with vulture form, which provides 33% disease resistance, and after gestalt, I recall it going up as well. I just can't remember the exact numbers on that one. It's been too long.

And the 20% Energy resistance does not get carried with gestalt metamorphosis by design, so that's fine.

The code and comments in Zulg's resistance function seems pretty deliberate in that you get the higher (better) of the two and not the sum of the two.

I assume that's the "... able to retain some or all of their..." part.

Then why do they add together in the case of a form vuln instead of the racial resist completely replacing the form vuln. By that logic, the form vuln (see vuln cold for diamondback) should be replaced by 20% resist cold from svirf.

Looks pretty clear that it's designed to give the benefit of a resist, so long as the resist isn't stacking on top of more resistance. So vuln + resist = partially mitigated vuln. Normal + resist = resist. Resist + resist = resist, not near-invulnerable.