In an unmodded game, only non-magical metal weapons (i.e. longswords, or axes, but not staves) can be affected and break randomly. That makes it worth carrying a staff on all characters until you get them magical weapons (some, like the longsword +2 from the bounty hunter near the mines or the warhammer +2 from Bassilus are available really early).
Do note that this happens even outside the mines, so there's no reason to hold off going there, just make sure to have a few spare weapons going in.
And no, Montaron and Xzar don't have special dialogue in the mines. IN BG 1 NPCs rarely have anything to say.