In all fairness, there’s a huge difference between kissing someone and sleeping with someone. I think a better analogy would be Tanya flipping out when Stacia went with Chris to the school in Forced Ascent. Just sayin!
If you think about Stacia’s behavior, her falling for Declan is consistent with her behavior in Demon Driven.
Chris saved her from a monster and she starts to have feelings for him. In a later book, can’t remember which one right now, he fights three werewolves to the death for her and it cements in her mind that she wants him. The fact that his blood getting in her wound creating the beginnings of a mate bond didn’t help.
Enter Declan. She spends some time with him in College Arcane and they hit it off during training. After all, she’s lonely and an outcast, just like Declan.
In God Hammer Declan saves her from a monster, and they spend all summer working together and she gravitates to him because Declan and Chris are the same character type.
As far as Caeco, yeah. I liked her character, but her and Krupp turned into raving bitches almost over night and the whole thing was weird.