Tried with the tree monsters but when I change them to Female gen name they show up as Chapter 1 Ramza sprites .
you're best bet is to use not-monsters for this. I'm talking things like Byblos, Ultima Demons, and WORKER 8, where they're built like humans. Set as female, adjust stat growths to match female, and you should have a female (unpoachable) monster. Not 100% sure how it'll interact with the job wheel, though.