If I have a party of 6. Let's say they are all lvl 12. Then I drop one of them out and import a brand new lvl 1 character. Then I go kill an orc, which my lvl12 guys would get 0 experience from..

Does the lvl 1 get experience? Or does it share the one pile of xp (from Orc prolly still none?) always to all member equally? I think the latter is correct, but now I am not so sure. For example in my party of six lvl 12 characters 4 of them are without race balance penalty, but they have noticeble xp differences. Like if they had been given bit different amount of xp in some fights. Maybe when one of them has been leveled and others were on the edge?

To pull it further.. would it be tactically good for your party to keep a low level character or more in your party who would drop the average party level down? Four lvl 12 and two lvl 1 characters = average party lvl ~8? That would mean my lvl 12 characters would get lot more xp than they would with lvl 12 party members. Would this xp gain be worth trying? And it could actually be pretty fun from roleplaying point of view.. having party of 2 masters and their apprentices etc. Any thoughts.. feel free to spam.

XP given is based on average party level, so having a 1st level with a party of 12th lvl might increase you xp awards. Hafta try it and see. As far as your other questions, I find that a good, blanced party, with all characters of equal level, is best. I played with 2 human fighters, a human paladin, human cleric, 1/2ling rogue, and a moon elf sorcerer. The paladin and rogue each got a level in ranger for dual-weapons, but other than that pretty straightforward. I've been playing D+D since 1978, and I have always found that this works best in the long run, whether it's played on a computer or a table top.