Your thinking way to small I’m thinking in the tens of thousands and your thinking in the tens lets just say for the argument that there 4000 Horde warriors with various bits of power armour and various weapons with no real training to 1000 properly armed and equipped NCR soldiers. Starting to get the idea?They're throwing sticks and rocks, while there are many fewer people with miniguns, organized militarys(think the NCR not too long after Fo2), and tons of basic weapons. Play though Fallout, and just look at the stockpiles of stuff you get. 12 miniguns, 13 rocket launchers, 2 gatling lasers- Just from the military base. It's an insane amount of equipment that 'civilized' people have. Put a few people in Power Armor, give them Gatling miniguns, and numbers go down.
I realise that, and as i said before the leader needs a secondary objective.It's not good for a main villian, because of the fact it's just not practical. It's black and white, and isn't very multisided. The Master and Enclave both had noble goals, although the Enclave was a bit more twisted in their was of achiving such goals. The Master hid himself behind religon.
Ok what if the new leader was a Vault Empire spy (i just made them up) that is trying to play The Horde and NCR off on each other (force them to fight) so that when all the dust had settled they could invade with out risk of failure
Ok then lets expand on the idea of clans starting from biggest to smallestThese tribals just want to wipe everything out. Not to mention their social structure couldn't support anything very large. Cheiftans are nomadic and small by definiton, and so unless they developed states, nonslash-and-burn agriculture techniques(leading to agricultural surplus, and that leads to specilization, and so on, and so on...) they'll remain small, and relitively weak when compared to most towns, cities, and brotherhood outposts.
1) Warrior Clan the biggest and most powerful clan in The Horde most people have no other choice since there's no form of education available so they have to fight to survive.
2) Techies If your to week to Join the Warrior clan and survive past childhood (a difficult thing to do) you usually join the Techies. They make hammers knives etc and can do basic repairs to guns.
3) shaman Doctors etc. If you do any thing that is considered a miracle like bringing some one back from the dead you are made into a shaman the only medical treatments available are bleeding and trephine (drilling holes in the head)
3) farmers Not exactly a clan in the strictest sense its mostly made up of slaves working on farms since people in the Horde know nothing about such things. the only free members are really the slavers that control them.
Let me expand a bit on the slaves. Slaves are taken from ether other tribes or in the raids on the "civilised" south Its very rare for a slave to be taken but when one is it’s sent strait into farming.