I'd say, rather than having goblins unable to harvest or craft their own stuff, (which would be silly) instead give them a need to fight, so that you have to lead your gobboes to war to prevent them tantruming from boredom and killing each other.
That sounds cool. Goblin tantruming would be ten times as bloody as Dwarven tantrums. But I still stand by the "goblins can't make everything". They need to raid and pillage other settlements because they cannot make some stuff themselves. In sieges and ambushes, the goblins don't have top of the line armor and weapons right? They can create very basic tools, armor and weapons, but if they want the better stuff, they're going to have to take them off a dwarven corpse.
What about kidnapping children? If you kidnapped, say a dwarf, said dwarf should be able to forge dwarven weapons.
Not at all! It's not inborn, it's cultural. Nature vs. nurture. Of course, kidnap him when he has learned his skills already and he still can use them, or develop them individually, but he cannot benefit from dwarven society since he's not a part of it anymore.
Different civilizations of the same race also ought to have different cultures.
Not exactly. Forging, crafting, that sort of stuff, is something a Dwarven has a feel for since birth. Once they grow up, they can start learning, and learn quickly. This is something goblins should be incapable of learning. Goblins should craft, only their items must be significantly inferior to that of the dwarves. They can only steal dwarven children or babies ( child snatchers ) and those kidnapped kids are brainwashed into goblin society. Despite this, the metal still calls to them, and they can very much learn to forge, and do other dwarfy things, for the goblin cause.