Wow.....a Nalia bashing thread here I see.
Personally, I think your party is ok, it is similar to the first party I ever had except the pc is a cavalier. It is fairly balanced since you have 3 tanks and 3 spell casters.
As for the details, honestly I would pick Nalia over Cernd any day since you already have another druid in the party (i.e. Jaheria). Now, if you can get over the annoyance of Nalia's comments (which is where most ppl have trouble with), she is actually a pretty good mage. She has 87 in stats, which is the same as Imoen but just below Keldorn and Jaheria. But in the 87 she's got same INT, DEX, and CON as Imoen...and she also have higher STR (14) than Imoen....but she is low in WIS and CHR. However, since she is a mage, we don't really care too much about WIS and CHR, your pc should have plenty of CHR for conversations. As the thiefing of your party, both Imoen and Nalia is decent when it comes to detect/disarm traps, but if you want to do other types of thiefing...then you might want to consider Jan for either one of them.
I would not recommend Korgan, mainly due to the role-play and alignment factors.....Korgan and Aerie don't (and I mean DON'T) get along, since you like Aerie, Korgan is out of the question. Having a rep at 20 is also problematic since evil characters tends to leave when rep hits above 18. If it is another pure tank you want, then you might want to consider Valygar or a range fighter like Mazzy (she is VERY good with short bows you know....but short swords??).
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Long live H22A1!!!!!