Hi Alex! Wow yes you do. 😄 Millicent is going to be an interesting one - I can see Theo having been an asshole to her in their earlier years at school, though once he got a grip on himself he came to see her better qualities - loyalty and blood and the potential that she might be useful to him (yeah, in some ways he's definitely still an asshole) so he's probably been polite and friendly to her since?
Looking at the rest of your list, I am very curious about Marcus Flint! He and Theo seem to be similarly ambitious so I feel like there's an opportunity for something complicated and antagonistic but mutually beneficial there? I could also see Theo and Hermione being weird old library buddies or similar? And obviously with Theo as personal assistant to Draco, he will have come to Narcissa's attention and probably interacted now and then.