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Can the crimson slaugther book freely take any FW choices that are available to base CSM (Fire Raptor, Relic Sicaran). OR are these choice only available to Base CSM?

You can take them. Crimson Slaughter count as CSM for the purposes of allies and other inclusions.

The reason for this is due to the word 'supplement' found within the title and the below rule:
In the case of codex supplements, the Faction of all the units described in that publication is the same as the codex it is a supplement of.
-Level of Alliance

The Rules are now being written to limit particular Units to certain Factions, which allows Supplements to also take said Units as they posses the same Faction as the mother Codex. Now you do have to be careful to make sure the Rule states 'Faction' instead of 'Codex,' as it is an important key feature. Rules that single out a specific Codex will still be able to create problems, as they are focusing on a specific book and not the Faction as a whole, but likely will be updated to match the new system as such Rules would be out-of-date. I could not verify exactly what the Rules state to give a more accurate information, as the Forge World site is undergoing some maintenance.

I would assume that crimson slaughter could take any FW stuff that CSM can take.