FINE ILL MAKE IT TOMORROW
15 characters in all, best ratio would be 8:7. I'm thinking about separating Shizuka/Naki into different groups, and For Kuchina, it'll probably depend on how the characters are placed.
From what I've gathered so far from character bios/roleplaying (which isn't much):
Kimiko
Yuki
Serena
Shizuka
Naki
Rikane
Trinity
-----
Aoi
Nozomi
Haruhi
Takene
Asumi
Honoka
Kuchina
Suzuko
Honoka
this is one way to group it personality/history-wise. Yuki/Serena have a close bond, Yuki is Ramone's tutor. Karen, Kimiko's pixie, was the first to interact with Trinity. Nozomi/Haruhi are a pair I personally would not separate due to age and stuff, but that is completely Oliver's call. Takene and Asumi probably wouldn't split up, though it would be interesting if they did. Kuchina would probably join whatever team Asumi or Takene was in, and because Suzuko has a sisterly affection for her, she may follow.
AND I WAS GOING TO PUT NAKI AND HONOKA TOGETHER BUT NO ANIMATIO I LISTENED TO YOU AND SPLIT THEM APART
This grouping MIGHT work the best personality-wise, and there would be a lot of trust since most of the characters in each group already know each other.
But that is just my understanding of your characters, so it depends, and you get to decide how you want your characters placed. Not me.