can the 12 true signs appear in other blood castes other than the one they're the true sign of? i think that'd make some interesting trolls

Nope, signs are caste-specific. They’re ways for people to be identified, so they should give information on the caste and associate to a specific name. If you went out with, say, a green Scorpio symbol, you’d likely get spotted immediately by a Drone as a caste mismatch and very possibly executed for hiding your identity.