Q. 62 - Can Multiweapon Fighting be combined with Natural Weapons? For example, if i had 4x tentacles...
Nope. That's what the Rapidstrike (Draconomicon) feat chain is for.
If that's so, how does a Monstrous Humanoid qualify for it? Any feat or maybe a Low-LA Template?
Any template that changes your type to aberration, dragon, elemental, magical beast or plant.... which is to say half dragon and not much else.
Aberration:
Half-Illithid, Spellwarped (not applicable to Monstrous Humanoids)
Elemental:
Elemental Creature (Air, Cold, Earth, Fire, Ice, Ooze, Smoke, Water, Wood) (All spread out between MotP and Dragons)
Magical Beast:
Shadow Creature (MotP version)
So there's a few more.