The charm will only be broken if they do something obviously hostile to Gogmurt. If there's a 4v4 fight as arranged, that's fine. And goblins are always fighting and doing stupid stuff. So until the charming PC does something that's seriously out of character (by goblin standards) for a friend of Gogmurt, it should hold. That won't stop him fighting the other PCs, though.
Otherwise, if the goblins are full of beer they'll suffer various penalties. I can't find any specific rules immediately, but -4 Dex, -4 Wis and an even greater tendency to do stupid stuff sounds likely. It could just turn into a huge bar fight with half the goblins beating each other up (not fatally, but enough to keep them out of the way). So you can balance it as you like.
The problem might come if the noise attracts someone else, but that's in your hands.
If it were me, I would not try to remove goblins from the fight. The PCs brought them all together, so they're gonna have to deal with the entire crowd. If that means PCs die, then they die. Starting a fight in the middle of a goblin tribe, surrounded by dozens of goblins, means you picked your battle very unwisely. Artificially limiting it to a 4 vs. 4 fight (or something like that) would be lame.
This setup should be the story of how heroes die.
Having said that, as a GM who would like the story to continue, I would certainly look for ways to give the PCs a chance to survive. And in Rise of the Runelords, there is a HUGE way:
You should take care to present these goblins’ tactics in battle as scatterbrained at best. One goblin might try to clamber up onto a nearby table of food (Climb DC 5) so he can gain a +1 bonus on attacks for higher ground against a PC. Another might get distracted by a plate of salmon and waste his action stuffing his pockets with food for later. A third could grab up a big carving knife if his dogslicer breaks. Each time a goblin takes an action, he should interact in some way with the environment, even if doing so wastes an opportunity to hurt a PC. The point of this battle isn’t to test PC resources but to set the scene and flavor for the insanity that is the goblin.
The module has made it clear: goblins are chaotic balls of insanity when it comes to combat. They don't even necessarily do the optimal things in combat. If you have, say, 25 goblins in the fight, then it's possible that the breakdown of how it plays out is like this:
- 4 to 8 goblins engage in melee with the PCs. These are the real dangers. You can slightly gimp these goblins by having them use an action to draw a weapon, but since they don't get multiple attacks on a full attack, this won't really eat into their # of attacks. But every chance you find to have the goblins waste an action, go for it. Also, do not have them flanking unless it's easy for them to do so.
- 10 to 15 of these goblins can resort to their bows. These should almost never hit. Why? Because there is a -4 penalty for shooting into melee, and a -4 penalty for shooting through any square in which an ally is in the way. Those penalties stack (and you can remove those penalties by taking Precise Shot and Improved Precise Shot). So allow the melee goblins to block or get in the way of the archers. Archer goblins are normally at +4 to hit with a bow, but with both penalties, they'll be at a -4 to attack rolls. Basically they'll probably need a natural 20 to hit.
- Remaining goblins: agents of chaos. For example, maybe one of these guys is an archer but instead of taking the shot with penalties, the little guy gets angry that the other goblins are in its way and it literally drops the bow, and uses a combat maneuver to shove a goblin out of the way, and then picks the bow back up and takes the shot. If it works, the goblin doesn't have a -4 penalty to his/her shot, but in the meantime the goblin is just wasting actions left & right, trying to line up the perfect shot. Other goblins would do similar action-wasting things. One might rush downstairs to alert the other groups. One might rush outside or through the fort shouting for backup, not realizing that all the goblins are already in attendance. Some might be utterly confused, wondering who this false Tsuto is. Some might even ask the fake Tsuto what to do -- and he might have a chance to turn some goblins against the others.
There could be more ways to have this go haywire. I recall reading that at least one of the goblins was a turncoat -- maybe even the charmed Gogmurt was the one I'm thinking of (this is the goblin who was staying outside of the fort because he didn't agree with the tribe leader's decisions to fight the humans). This goblin could turn in a fight, swaying the odds in favor of the PCs.
I'm sure you'll think of other things. The point is to keep the goblins, have the chaos, enjoy the massive, massive fight, but find little ways (ridiculous ways?) to have the goblins be there & fighting but somehow less effective than they should be. Try to keep it plausible. The players will know if you are just ruining the goblins to hand them the victory.
Good luck!
First off, there are barely 25 goblins in the place and that assumes you include the 10 Birdcruncher refugees. Where will the "Battle Royal" take place? You could make things simpler by having it in the Exercise Yard (C16) and have Ripnugget order the Birdcrunchers to stay behind because he hasn't accepted them yet.
That leaves 12 goblin warriors, 8 goblin dogs, 5 commandos, a warchanter, Ripnugget (and his mount.) I'm assuming the NPC's from Dungeon Level 1 (such as the goblin wives) stay there. That's still more than the pc's can probably handle at once. You can have Ripnugget put some in the watch towers and maybe put some goblin dogs back outside.
I'm unclear who is going to fight in the battle - there are 5 pc's? One disguised as Tsuto? So four pc's vs Ripnugget, Gogmurt (who's charmed) fake Tsuto and a befriended goblin? And that means Ripnugget doesn't have any allies in the fight? Do the players have a plan? Are they going to participate in this fight? If they do and it becomes immediately clear some kind of trick has happened as everyone turns on Ripnugget that'll likely force a fight with all the witnesses as the chaotic goblins are going to struggle to respect any "sportsmanship" 4 vs 4 limits under the best circumstances. But what if fake Tsuto is made a representative for Nualia by Ripnugget and replaced with the warchanter and the goblin friend who's already proven himself a hero is replaced with a commando? And the then the pc's are smart enough to focus on Ripnugget? If they defeat the chief, they can take over, supported by Gogmurt who doesn't like Ripnugget's alliance with Nualia in the first place. And even the fake Tsuto could support Gogmurt and the pc's. Or they could put Gogmurt in charge and he makes a deal - they can access the Dungeon and the longshanks that are the real problem as long as they leave the rest of goblins alone.