You can equip them with monk weapon prof (no extra feat for exotic needed) but monks only get their special ab progression with kama, goad, nunchaku and quarter staff (provided you've got 21 monk levels). Those are all 20x2 weapons (just as unarmed strike) for balance reasons I guess. The main reason for implementing goad and nunchaku are base damage types (again, a guess), as kama is slashing, goad is piercing and nunchaku is bludgeoning.
Ah I didn't realize that. I guess my Sai monk assassin isn't going to work. I think monk progression might be extended to some other weapons eventually, but perhaps not to anything with a crit range larger than 1.
Have you verified that? I thought Kama, Goad, and Nunchaku were the monk weapons, with QS and Shuriken added at 21 monk levels.
Monk grants proficiency with Sai, Goad, Kama, Nunchaku, WFW, and Shuriken (plus dagger, katar, handaxe, club, QS, sling, and crossbow), but that's different than having monk attack progression with them.