Spoilers
Not exactly true, experience says that certain combinations of characters can be a lot of fun, so for example having minsc and jan in the same party cvan be a barrel of laughs (and they are pretty decent npc's), whereas maintaining a lawful good character in conjunction with Korgan and Edwin will be impossible. If your a male character, getting Aerie or Jaheira will have a good romance angle, while Viconia will be intense but riveting (the bit where she tells her history is scary). If your female, the only male character willing to have a romance is apparently Amoen, though Haer d'alis can be a bit of a jack the lad...
In my humble opinion, for a necromancer, useful npc's would be...
In terms of tanks....
Keldorn (Paladin - sewers)
Minsc (ranger - start)
Jaheira (fighter/druid - start)
Cleric/mages...
Aerie (Cleric/Mage - circus) or....
Nalia (thief/mage - Copper Coronet)
Amoen (Fighter/cleric - CC)
Thiefs...
Jan (Thief/illusionist - government district)
Yoshimo (bountyhunter - start)
Mazzy (thief - Umar hills)
I have listed 9 npc's and you only need 5. (hope I haven't left anyone out, I'm doing it on my fingers here) I have excluded certain npc's (haerd'alis/ cernd/Valygar/ Edwin/Korgan/ Viconia) as being either unsuitable or plain dull (though haerd'alis has his moments and the adoption of Vagyar for a while will probably be necessary in the later game, figure it out as you go along
My own personal five would be (and do remember this is my own opinion)
Minsc
Jan
Jaheira
Keldorn
Yoshimo
purely for storyline and comic relief....