There is a theory Natsuo is quirkless and can't cool himself. Its unconfirmed, his character page does not list a quirk. Fuyumis quirk is confirmed as ice, but Last I checked Natuos was blank or unknown.
Its unknown if Rei or Fuyumi are weak to heat.
Fuyumi does have the spots of red in her hair, implying her constitution is closer to shotos.
At least in Dabi's case I think it has to do with the fact that his skin cells probably expressed more of the gene that would make him resistant to very cold temperatures but not hot temperatures which is his quirk and why he is able to burn himself, i.e he got his mother's skin but his dad's quirk, not a pretty match the two make. Quick bio lesson(though paraphrased) the same parts of you body can express different genes! Which is one reason some people may have blotches of different color skin and also is the reason for heterochromia(2 different eye colors) which is a staple in anime~ i assume it is also the complete explanation for Todoroki being half n half, though yall probably already knew that already~ i recommend looking up cats that are half orange and half black, it is really fascinating!
I'm kind of wondering why if the ice quirk Todoroki's are weak to heat (you remember Natsu being hugged by Endeavor) than why aren't the fire Todoroki's weak to cold. I mean like if the fire todoroki's can just warm themselves up when it's cold then why can't the ice Todoroki's cool themselves down around heat.