I wouldn't be too eager to apply Gomi as a defense against internal burning myself. He's heat resistant, not heat conductive as far as I'm aware, so it'd be more of a scenario where someone wears a firefighter's uniform, and then set themselves aflame inside, thinking that the uniform would protect them. Not like the Gomi slime literally fuses with your body, right?

Pretty sure the slime being an exterior coating would actually interfere with Sanyu's ability, which I think is contact based?