First, a sixth slot should be flexible for any companion quest you would want and still have to complete.
Alignment is something, I always regard, no matter the "balance". I try to choose my companions by taking only those with the same alignment as my Gorion's Ward or one that's a direct neighbor to it. Take a look at the bard class infobox to see what I mean: the alignment grid there shows exactly what for me would be possible if playing a true neutral character.
The wanted romance, of course, plays a role, and then party members' affinity among each other. And how much you like them. (Or need them, due to your own play style.)
You – (and if I'm not wrong, you forgot to sign in, Crossbow3 ;)
Jan – huh? second mage/thief? yes, you will need one, later …
whomever you want, as there is no "ranged warrior" in that line-up; perhaps Aerie or Viconia
What do you mean by "have used the following characters thus far"? You haven't had e.g. Nalia or Keldorn in your party, yet? Then you have missed something … And there are more, outside the city …
Do you play non-enhanced?
Oh, just happy to have someone listening (reading) to my blabbering. ;)
Okay, I have to correct me: there's no real need for a second mage/thief – I forgot a little detail from the plot.
I do not always restrict me this way when it comes to the alignment. For example, if I play a ranger, he usually romances Jaheira, while being of chaotic good alignment – that doesn't match completely my pattern. And there might be more exceptions.
I don't know if they're all complete, but most party conflicts are listed here on the companions' pages. Or you prefer to get surprised. ;)
And of course I'm thinking of the – sort of – permanent party. The companions I'd like to have always around me, after the possible quests of them all are solved. Sometimes, however, I choose by the quest – take a ranger with me when visiting the woods and swap him for a thief when in the city, e.g. Or if I have to expect undead, I perhaps strengthen my clerical power.
When I have to choose who temporarily has to leave to make place for a quest companion – is that what you mean by "who would be the 6th rotated out for companion quests"? –, I consider their role in the party: Comes a thief? Goes a thief. Comes a cleric? Goes a priest. Generally. There might be exceptions, depending on the actual situation.
Sometimes quests are tricky, and evolve completely different according to your party compilation.
One very important thing: before making the last steps in the chapter three main quest, you should have visited every location possible, taken every quest you want to take, and tried every companion you would like to see in your party.
By the way, some people play with three mages in the party – just for their pure power in the later game.
Yes, I meant the 6th guest as the one leaving for companion quests.
This game is so overwhelming in the number of choices and options. I'm basically using this playthrough as a learning experience because there is no way I can take it all in. I am playing BG2:EE for your information, I'm just sort of clueless when it comes to details.
Whatever advice you have to offer I appreciate, even if I can't use it all or take it all in.