With respect of course, Nlinindoll, I agree that the bottom line for HeroQuest is that like classic D&D, Warhammer and many other games, within the core of the rules is the ability and privilege for the GM (Zargon/Morcar/Grimdead) to modify things without needing to ask permission of anyone, though his choices may impact the tone and enjoyment of the session so he should choose wisely. That said, the bigger issue is the question of using assets beyond what's provided in the GS + that expansion.
Kellar's Keep is a bit infamous among "classic" Heroquest for introducing cheap, wild gimmicks that younger players and sore losers (of which I was one myself in my youth!) may find unfair and even complain about... with Zargon's natural retort being "that's how the quest was written!" (even though sitting in the background is the bottom line that "the GM is always right... and if you don't like it, then when it's your turn as Zargon, you can run things differently!").
Kellar's Keep was released in 1989 (NA edition in 1991). At that time, there was no indication that they had built in the rules of future expansions, so it is most natural to assume that they were only taking the Game System and said expansion into account with this note.
Why assume that? Kellar's Keep was first released in Europe, with the NA edition being a revised localization. The point of the 2021 remake of HeroQuest was to make an aesthetic adaptation that was basically a 1:1 re-creation of the North American editions of the game, with the only revisions being to necessarily replace any potentially IP infringing aspects of Warhammer Fantasy owned by GamesWorkShop (and a couple of minor nods to modern notions of political correctness, let's face it). Thus, there was no attempt to incorporate ideas from other expansions or even the Mythic materials. It's as if those other materials don't exist at all when playing this.
Now then, so you own the Mythic tier? So do quite a few of us at Ye Olde Inn. Do you need a good excuse to incorporate that material into the game? I actually sub in the alternate minis quite often, and I toss the unused artifacts into random places to spice things up, but that's my choice. There's nothing in the books that says you can or can't do that, that's entirely up to you.
BUT if you were trying to guess the intention of the original designers, you'd play it as written. Anything else is homebrew... which is fine, but I think the question being asked was what was the intention behind the quest's notes?
You may choose any monsters except the following: the
Chaos Warlock, the Gargoyle, 1 of the Zombies, 4 of the Skeletons
and 3 of the Goblins.
I own dozens of 3d printed models, proxy miniatures from Reaper, and multiple HeroQuest sets. I guess this means I can throw entire armies of creatures into the fray, because it doesn't say I can't! Okay, that's literally true, I suppose, but was that really what the writers of this quest probably reasonably intended? They also didn't say I was armed with a double barreled shotgun and a horse made out of cotton candy. I'm free to introduce those into the game at any time, but it's probably not something the writers of this quest specifically intended.
So I think the most natural reading is that they intend you to use up to 9 goblins, 16 orcs, 2 Mummies, 4 chaos warriors, 6 fimirs, 1 Zombie, no skeletons, no chaos warlocks and no gargoyles.
Why limit the monsters at all? Presumably its done to keep things dangerous but not overwhelming in difficulty as they try to hold off the army. If you start throwing Polar Warbears in there, you might start killing heroes and swarm them for good.
Space Crusade does a similar thing with its use of reinforcements and blip tokens. They don't state precisely how many assets they assume you have, but tell you to leave out certain ones in certain places to limit the difficulty. But wait, what about the verbiage "of the.." doesn't that imply there are MORE than 4 skeletons? At best they are assuming you have Return of the With Lord here, and thus we revise the total above to 6 Mummies, 5 Zombies, and 8 skeletons, which can make it quite a bit harder, but still not as tough as it would be if you also threw in another 2 goblins, 2 orcs, 2 chaos warriors, etc. (note "the gargoyle" implies there is only one, hence no using the bonus one from the Mythic tier that didn't exist when Kellar's Keep was written). But even with minor tweaks the effect is the same, a virtually "unending" horde is sent after the heroes, and they won't be able to stand and kill ALL of them, as that's not the point of the quest. Very little changes if you start introducing a few extras unless they start thinking maybe there is a chance to reach an "end boss" in the group or something if the group is determined to keep fighting all the invaders.
The first time we get a real acknowledgement and even encouragement to use OTHER expansions at the same time as the one you're using within the realm of HeroQuest is EQP/BQP (Mage of the Mirror and Frozen Horror respectively). They acknowledge that since the Alchemist Shop only offers one potion that isn't exclusive to one hero, that you should also make the potions from OTHER expansions available as well, without specifying which ones they mean. A similarly revised rule in the remake edition of the Mage of the Mirror says you can use the Mercenaries from other expansions (such as Frozen Horror) in that pack's group quests, a rule that was previously never mentioned. The Mythic tier stuff assumes you own everything within it but I wouldn't apply those rules retroactively from the newer quest books, unless you want to, back into the quests that were written three decades before the Haslab campaign began. Some older rules make less sense in context of the Mythic tier anyway, for instance the notion of the "large sword orc" (as if there is only one, when in fact four such orcs exist in the Mythic set) in two of the early quests to represent Ulag/Grak.
If you want an excuse to introduce your mythic pieces into the older quests, I say, why not? But it's a bit like incorporating the Warlock into the storyline of the game system, it's not something the original designers ever intended. It's not WRONG, it's just homebrew, so it's irrelevant to the question of what the designers probably intended and it will change the experience. Results may vary.
Zargon is tasked with making judgment calls when players make unusual requests, or glitches or exploits are found in the game, or even oversights of the quest writers (how were they to know the details of expansions and updates that weren't written or even planned at the time they released their pack?). I love that these forums serve as a sounding board for those suggestions, but ultimately the decision is yours, not mine or any other board members, so don't read what I'm saying the wrong way of course.
A rule of thumb I'd use is that if the heroes are getting bored and it seems too easy, be a little more liberal with your additions of monsters. If it's taking too long, the frustration is high and they're struggling, be more conservative with your expansive ad-libs. Are the heroes confused about the mission in this quest and just taking forever to try to kill every single monster (and getting lucky rolls while at it)?
TL;DR ? So in this case if I were Zargon I'd limit myself to GS+KK and assume ROTWL assets as well IF it was "too easy," at that point, but stop there. Ignore my example if you wish, of course.