A nice, clean way to do it would be to split the 3 Holy Symbols not along the basic Good/Neutral/Evil lines, but according to the specific alignments used in Cleric Remix or the like. If you're NG, CG, or TN, you get a Symbol of Lathander, all Lawful alignments get a Symbol of Helm, and CN, NE, & CE get a Symbol of Talos, or something very like that setup.
Having Anomen get his Symbol through a quest and not a simple level gain makes a great deal of sense, earning his god's favor and all, and would also prevent the annoying glitch of multiple Holy Symbols gained as a byproduct of Petrification, Imprisonment, or booting him from the party. The drawback, though, is that if you write a quest for Anomen, then Aerie and Viconia have to get one too, and they all have to be different. Another critical consideration is just when and where this quest is initiated: Suppose Anomen hits Level 25 right after the party is sent to the Nine Hells. It'd be kind of silly to ask him to go to the Bridge District and uncover new evidence regarding his sister's death at that stage in the game, and a surprise ambush by Priests of Beshaba (with no explanation as to how they got there) would be just as nonsensical, given the circumstances.
It's a good idea, but must be implemented very carefully.