Not all of them fight vampires. Greg will I believe. My created weresim didn't engage in a fight with Lilith Valtore but upon meeting they had half negative friendship. So it is instant. It didn't queue up to fight Lilith but I also sent my weresim home.
I haven't noticed anything with mermaids or aliens yet. My spell casters can't do magic on Greg unless the friendship meter is about half or so. Not sure if they're somewhat limited in magic at first or not yet or if it's specific to Greg. I haven't been through enough moon cycles or sims to say just how much lore overlaps but there is some for spell casters and most deals with vampires.
Greg was a spell caster before werewolf. Greg seems to have a particular hatred for Vlad that goes way back. So from this I don't think mermaids and aliens are included in the back story or lore.