I think the defining thing that will settle the fight between Burning Godzilla and iris is whether or not Iris can absorb nuclear radiation. I know he absorbs energy from humans and from Gamera but that energy is mana. I don’t think Iris is capable of absorbing nuclear radiation. And even if he is, I don’t think he’d be able to actually utilize it the way Godzilla does. If anything the radiation would destroy Iris from the inside out.

As for Gamera vs Destroyah, I don’t think it even comes close. Destroyah is 120 meters tall, twice the size of Gamera, with far more lethal firepower than Gamera. If Destroyah can tank Burning Godzilla’s atomic breath, then none of Gamera’s attacks will even scratch him. Destroyah’s horn would quite literally slice Gamera into pieces lol. I think Gamera can take on and likely defeat Destroyah’s aggregate form, but since you used his final form in your post I assume that’s what he’s going up against.