Aerie: A multiclass Cleric / Mage. Which other character / race can multiclass Cleric / Mage? None, so you have to choose her.
The main character can, provided shs's a half-elf. Or if she's a gnome, provided you don't mind having to take the Illusionist kit (more spells/day, but you lose access to Vampiric Touch and Horrid Wilting).
Also, if you happen to find an NPC named Quayle and cheat him into your party (Control-Q cheat), he will actually be a Cleric/Illusionist. (He is a normally recruitable character in BG1; the developers did let him keep his class in BG2; don't know if his stats are the same.)
Imoen: Your second mage. Either Nalia or Imoen can wield the Dak'kon's Zerth Blade +2 and a defensive blade in the off-hand such as Arbane's Sword of Agility +2. The other character can use the Staff of Magi. The important thing is to have access to another mage's arsenal of spells.
Note that, in the party you gave, Imoen will be in charge of finding and disarming traps, as well as opening locks. Before you get her, you can use Yoshimo as a substitute for that role (though he does not get spells), or you can try to use Nalia's inferior thief skills.
(Of course, this assumes that you aren't going to skip Spellhold with a glitch, but doing so is clearly not intended and will skip important parts of the story.)