Are there any specifications for what kind of lusi different blood castes get? And do they have to have a relation to their troll’s sign? (Deercat and Dammek, or Aradia and her Ram lusus for example)

Not necessarily? There are some apparent things so far but not too much, like. Aradia had Ram Mom and Xefros has a Sloth, both are Mammals so there’s that. On the other hand, Vriska has Spidermom and Ardata has a Tick, so Ceruleans may be more Insect-based. Tinkerbull and Deercat are hybrid mammals. There MAY be some pattern here, but we really. Don’t have enough examples yet to tell for sure.