I joined up with Monty and Xzin, but they attacked in the Friendly Arm (I forget whether I had picked up Jaheira and Khaled yet.) and I whacked them without resurrecting them. As a newbie, perhaps I was clumsy and caused it myself?? Are they gone, gone, gone?

LOL, the same thing happend to me in the Fieldpost inn. one of them said something about harpers being idiots to Kaliad and suddenly both pairs left the party and started killing each other. Monty and Xan got killed and i bumped them off the party without raising them. IMO i dont thing it really matters, they were useless compared to the other NPCs.

Yep--if you boot a dead person from the party, they are gone forever. And Montaron is far from useless, he's a damn good Fighter/Thief. Xzar is quite potent as well, it's just that having a low Constitution is a real drawback in the early stages of the game.

In both cases, having a high Charisma can postpone intra-party conflicts--perhaps even forever, if it's high enough.