The first six GF can be easilly assigned...
They work in couples Ifrit, Quezacotl and Shiva should be split up, and each assigned a partner. I think the best teams are:
Ifrit & Siren
Quezacotl & Diablos
Shiva & Brothers
3 of them have HP-junction (I think Ifrit, Quezacotl and Brothers), split them up!
Just look at their abilities, with the right teams everyone can junction a lot. Until you have these six you may want to assign them in an other way.
Squall is a fighter and should get Ifrit if I'm not mistaking.
Selphie is a magic user so she should get Quezacotl.

CORRECT ME IF I'M WRONG