Is this a good idea for a book?

Ah! Very cool! Don't change using "them" though. I like it. Just don't overuse it. You can maybe think up another name to call them as well besides just "them." Because it might get confusing if you use the word them to describe someone who's not a "them." Know what I mean? Lol but besides that I really like the premise! If it becomes romancey, then just let it happen, don't force it.