I was wondering if there are stories when Spike is a slayer along with Buffy like "Dancing in the Moonlight by Holly" or spike that becomes human but still maintains his vampire strenght like in the Chiaroscuro series?

I'm not sure but I think Spike is human+ in these...

Sideways Shanshu by Hlynn sequel Sideways Shanshu 2
Picks up right where The Body left off, and continues from there. Buffy has to try and deal with the aftermath, and it's just then that Glory decides to push things into overdrive.

Changing History by chosen
Can a man without memory find his way back to the woman who claimed to love him?

Cleared Path by Heather Martin
Takes place at the very beginning of Tabula Rasa. "We don't know what we've got till it's gone."


Spike becomes a slayer/human eventually (within the sequels I think) of these...

The Bunnyverse - Humping Like Bunnies by sabershadowkitten, Heart Don't Lie by nautibitz sequel At it Like Rabbits by nautibitz, Baby Likes to Roleplay by nautibitz
The series/saga that jumped off from a sweet (and naughty) little story about Spike and Buffy being rabbits for a night. Not as "fluffy" as it seems... Timeline (at the start of the series): An amorphous S4 AU where Spike is working with the Scoobs, Buffy lives at home, Oz is still around and Giles still works at the high school library. Also, Anya has yet to return. Which is probably a good thing, considering she'd be really freaked out about now.

Alternative Intervention by kittencaboodle sequel Intervention Aftermath
Goes AU after Intervention. Buffy, with the encouragement of Dawn, stops being such a cold fish and helps Spike after he gets the snot kicked out of him by Glory.

Also another human+ one...

End Of All Things by KittenOfDoomage
AU from Chosen. Spike shows up as a ghost as it happened, only this time, Buffy dreams about him and finally finds out that Angel has lied and he is alive. She heads to LA where the fun begins...

Again human+ but not for the main body of the story...

Memory Redux by madrog
Season 7-ish, goes AU. The PTB and a wish separates Spike and Buffy but will that keep them apart?

If I Was Your Vampire by MistressNobody

Spike's strength and power exceeds Buffy's in this one and you can find it on FF net