If it truly is a 15 person project for its entire duration then it's not worth splitting up, but you could still consider whether the whole team needs to be present for every standup. More often than not, though, when the team size reaches the 15+ range the project often can be broken down into smaller discrete deliverables that do not need 15 person teams to achieve.

Where I work it is all software. We can and sometimes do have folks on multiple teams but generally try to avoid that. And teams can definitely be cross-functional.