There's two types of tip - parallel and taper. The rule of thumb is that woods taper and irons are parallel.

No, if the shaft you're replacing is taper, you can buy shims to fill the parallel cavity.

That said, I don't know of any way to get a parallel shaft into a taper hosel.

But it's best to know for sure. Call the MFR of the heads you're reshafting, and ask if the hosel is taper or parallel.