Yeah, as much as Serena wants Hiro, the only in-character way I see that happening is if she gets betrayed from within the military and kicked out. And that would take extraordinary circumstances.
She's too good and dedicated of a person to abandon those under her, and Hiro will not let himself get tied down by anything that he isn't in charge of.
Can't say I'm a fan of harems, but I do think how it's been done with Mimi and Elma was believable. I don't think too many more will be believable, especially once things get past one per day of the week. The scheduling would be a nightmare.
For this particular setting, I'd say there's room (I don't agree, but I do see room) for a core group of women who are exclusive with Hiro and several who are into a casual relationship with him (some of those either not being into romantic trysts and being cool with an exclusive physical relationship and some actually seeing other men casually on the side).
So, like, Mimi and Elma would be core members while Mei being an android would be a character he occasionally does the deed with (should that option be installed). Some of the women in the future mentioned by spoilers might just be casual flings who work for him or with him on occasion.
Edit: Okay, Mei would definitely be more of a core member. But I think the concept of what I said still stands from a plausibility perspective.
I guess harems are one of those liberties fiction gets away with that IRL almost never work out well.
