In American Dad, is everything part of Stan Smith's heaven?

Probably not.

For one thing, the Christmas episodes tend to be pretty self-contained (for the most part -- Santa's animosity for the Smiths carries over into the Krampus episode, for instance). Notice that the angel that guides Stan to his personal heaven is very clearly Michelle, his lawyer from "The Most Adequate Christmas Ever", but neither of them give any indication that they recognize each other, even though Michelle has her wings.

The bigger clue, though, comes in the last seconds of the episode, when the camera pans away from Stan and Francine's embrace and shows Klaus' body mounted on the wall in the same pose as he was in Stan's trailer post-Rapture. Clearly, in Stan's preferred heaven, Klaus remains dead. Yet, in subsequent episodes, he is most definitely alive. It's not beyond the realm of possibility that somewhere in the back of his mind, Stan decides his world isn't right without Klaus after all, and thinks him back to life, but given his general dislike for the fish, it's not likely.

Though it's a fun headcanon idea to kick around, isn't it? And it's never, strictly speaking, been contradicted by Seth or any of the writers, so far as I can find.