Kingdom Hearts is completely different. Disney Characters has often been used in different roles, such as Donald Duck being tied into Ducktales, which is almost an Indiana Jones parody. You can take something that's inherently goofy (no pun intended) and combine it with whatever you want with decent results. You can even make it serious if you like.
But the problem with combining Silent Hill and Castlevania is that they're both tied to their respective locations. Every Silent Hill game has taken place in Silent Hill, or immediately around it. And every Castlevania game has occurred in and around Transylvania.
Castlevania has nothing to do with the Silent Hill universe. You could throw some SH enemies into Castlevania and it wouldn't be a problem. Look at Cthulhu and Yorick, which are both imported from other mythologies. But you can't combine their storylines or universes without making it goofy, because both series take their storylines too seriously. It might be harder taking Castlevania enemies and putting them in Silent Hill, because (at least in the earlier SH games) the enemies are tied to the psyche of the main character, and there is no reason for a modern-day Belmont or Dracula to travel to a small northeastern-US town to confront their inner demons.
Another problem is the gameplay. The series play in completely different ways. Even the 3D CVs have nothing in common with Silent Hill. How would you combine the two games without the gameplay being either completely un-CV or completely un-SH?
Both series take themselves seriously. Both series have their own, well-established universe. Those universes don't provide an option for accepting another universe's rules, locations, and characters. Unless you made a parody (like SilentVanius), it would come off as a goofy, uncanny mishmash, and it would not take itself seriously.
But if you disagree, what would you do to combine the series? What is your story draft? How would you combine the gameplay?