We know it's possible for people to be born with both the Inhuman & the mutant gene so why didn't we see any of them emerge when Terragenesis swept across the globe. Yes, their rare but we should've see a few Inhuman mutants being created if a mutant had their Inhuman gene activated by The Terragen Mists

The Terragen mists were harmful to mutants, that's the justification for IVX. Also being a mutant and Inhuman would put their movie rights in a weird place.

I think the only mutant/inhuman was crystal,and quicksilver kid right?

That was to be believed when we still thought Quicksilver was a Mutant and Magneto was his father. Since that's no longer the case, she's no longer a Mutant/Inhuman hybrid.

Also in the x-men books they come across Mister Sinister who tried to create a mutant/inhuman hybrid in order to eliminate the problem with the inhuman mist by way of a clone of cyclops, but it proved unstable and ultimately died

Ah wasnt that reveil apart of the high evolutionary arc a few years back. I honestly forgot quicksilver wasnt magnetos son...what is he technically now I though he was still a mutant.

He and his sister, Wanda, were "experiments" created by the High Evolutionary.
I can't remember all of the details off-hand, but in the Fox movies, they are still Mutants. In the MCU they are experiments of some sort.