They are in addition to the 20 goblins. You’ll find the rules for this on page 149 near the bottom of the first column.
I wouldn’t, but I couldn’t find anything in the rules on this one way or the other. You might consider that Bugbears are technically assigned treasure at the Warband level, which 6 Bugbears do not constitute. I’d likely place part of the Goblin’s Treasure Type E onto the Bugbears to represent their wages from the Goblin Chieftain. But it’s not going to break anything if you throw some extra silver on them beyond what the Goblins have.
Does anyone else break up each gang into its own room when a lair is encountered in a dungeon? Or is it generally assumed that you should be lumping all of the goblins or other appropriate monster into a single room and therefore one combat?
Lairs of monsters necessarily break out into separate rooms, for me at least. Squeezing 36 orcs, plus leaders, plus mates, plus spawn, into a 40 x 50 room is just ridiculous. Even if they felt comfortable packed in there, why wouldn’t they expand into the nearby empty rooms?
That’s not to say that a fight doesn’t often become a multi-room ordeal, though, as the fight draws attention or panicked foes run for help.
You are supposed to be break them out into their own rooms. From ACKS, Chapter 10, Placing Monsters:
Some types of monsters have lairs composed of a variable number of encounter groups. For example, goblins are encountered in gangs (encounter groups) of 2d4, and a goblin lair is composed of 3d6 gangs. If the stocking procedure has generated multiple encounters with a particular type of organized monster, then the dungeon should be assumed to hold a lair of that type of monster. One room in the dungeon should be chosen as the lair, and one of the monster encounters assigned to it. The other encounters rolled for that monster type should be placed nearby to form watch points, barracks, or splinter colonies. The various rooms should then be reinforced with any leaders, champions, or other creatures indicated by the monster entry. If space permits, the Judge can also add additional groups in other nearby rooms, up to the maximum number of groups that can be encountered for a lair.
EXAMPLE: Because the Judge rolled four encounters with goblin gangs, the dungeon holds a goblin lair. The Judge chooses a remote room in the rear of the dungeon as the lair, and places one goblin gang there. Nearby he places several guard posts with the other three gangs. He adds a champion to each gang, as instructed by the monster entry. The entry also states that a lair will have a chieftain, sub-chieftain, females, and young, so he adds those creatures to the lair room.