[houkoholic]

Actually you have to look at the current positions for Keishin first:
Airi: Centre
Maho: Power Forward
Hinata: Small Forward
Tomoka: Shooting Guard/Point Forward
Saki: Point Guard

It''ll be hard to move these 5 outside of their current position now that they are more or less specialising (except Tomoka). So when you swap in the 5th graders you have to keep that in mind.

Mimi is small but that doesn't make her good PG material, like Tomoka she's also too caught up on winning a 1on1 match up, actually she's worse than Tomoka when it comes to winning and not seeing the bigger picture. In fact currently Saki has no one to replace her except Tomoka (but only if Subara gives specific directions to Tomoka) so there's a weakness there (unless we discover something about the 5th graders later on to amend this). If Mimi is to come in I think she would replace either Tomoka or Hinata.

Kagetsu height and athleticism should make her a replacement for either Airi or Maho depending on situation, so she's the perfect candidate for training as a Foward-Centre.

The Takenaka twins would both be forwards, I don't see anywhere else for them.

Masami otoh is in an awkward position. Keep in mind there's no 3 pointers in mini basketball, so while Masami can technically shoot from the 3 point line there is no point advantage to it. She also currently has no other skills so it'll have to wait till later on to see which direction she would go, but considering that she's Saki's "rival" she'll probably end up with some PG skills and turn into a Combo Guard. For the rules of mini basketball currently it's better to move in for a high percentage shot and possibly draw a foul for a 3 point play in the process.