So, as a coda to this little thought-experiment, I gave some thought to monsters in a swashbuckling & sorcery 1e AD&D campaign.
Given that I'm not much of a fantasy fan, my own predilection is for a humans-only campaign with very few monsters. While AD&D can certainly do such a campaign, it also rather misses one of the reasons for choosing Dungeons & Dragons in the first place. So, the question becomes, include everything or limit the selection, and if the latter, how much?
My own inclination is to limit the selection; even when I ran bog-standard pseudo-medieval D&D fantasy, I pretty severely limited the range of monsters found in the campaign world from what was available in the various monster manuals. That said, it is personal preference, not an attempt to define the genre by way of inclusion or exclusion from a selection of fantastic creatures. If you want your swashbucklers battling ankhegs and neo-otyughs, more power to you, but I'm going to take a different path.
When I think of swashbuckling & sorcery, what first comes to mind is Solomon Kane. The world of REH's Puritan swordsman is filled with degenerate men, some wielding foul magics and trafficking with foul spirits, others just plain fouled by their own corrupt natures, but it's also a world with 'real,' as opposed to metaphorical, monsters. 'Dark powers' abound, represented in AD&D terms by creatures such as demons, devils, dæmons, and other evil outer planar monsters. At low levels the adventurers might find themselves beset by imps or quasits bound to the service of a witch, frex, before graduating to more powerful fiends as they uncover sinister schemes and attract unwholesome attention as a result. Rakshasas and ogre magi - oni are perfect for this as well. Appendix D of the Dungeon Masters Guide is the ideal tool for generating unique fiends for my swashbuckling & sorcery campaign.
In fact, unique monsters, or rare groups with a limited geographic range, appeal to me. More on that in a moment.
In addition to demons et al., undead are another logical category of monstrous foes for swashbuckling & sorcery. The whole gamut of horrors may be found in the game-world, consorting with the most vile and debased among the living. Revenants and ju-ju zombies jump out at me - so to speak - as particularly fitting: the venegeful, unquiet spirit and the horrific servant of a dangerous spellcaster.
Lycanthropes fit the swashbuckling & sorcery setting well, again tapping that notion of curse-spawned corruption among men.
Now, the common theme in all of these monsters so far is that they are found in concert with humans. For me, swashbuckling & sorcery doesn't summon up an image of a world on the verge of being overrun by legions of umber hulks riding armored purple worms - rather, most monsters of the campaign exist in the shadowy corners of the world of men. This opens up another door, that of the wee folk - leprechauns, brownies, pixies, nixies, and the like - found on the fringe of human society.
The world of Solomon Kane is more than just the dusty roads, looming castles, and dark forests of Europe, but also includes the wider world as well. In my conception of the swashbuckling & sorcery campaign, I would make a distinction between monsters found in mostly 'civilised' lands, and those found in the hinterlands of terra incognita. Beyond the pale of 'civilisation' is where we find intelligent apes, sabretooth cats, giants snakes and scorpions, and dinosaurs - 'cause, y'know, PIRATES FIGHTING DINOSAURS! - as well as legendary monsters like the sphynx, the kraken, the lamia, the leucrotta, and so on. This is where the notion of unique - THE Kraken, rather than a kraken - or rare, geographically restricted creatures comes into its own. Rather than 'vampires,' frex, a single vampire with a number of servitor spawn in a forbidding castle, dwelling on its accursed state, or a kaiju-like dragon turtle which haunts a strait in some distant sea.
Finally, there are demihumans and humanoids. My personal inclination is not to include either one, but, again, that really flies in the face of the game - why bother with D&D at all, then? With respect to the latter, what few humanoids exist in my concept of the swashbuckling & sorvery game-world are found in those distant hinterlands, in steaming jungles or impassible mountains, not threatening the civilised lands of men.
That leaves demihumans, which also impacts playable races. In short, dwarves, halflings, elves, and half-orcs are shuffled off to the HUMDRUM-V while half-elves and gnomes get to ride in the FUN-V.
Why? First, gnomes are fun-loving, not terminally stodgy like dwarves and halflings - plus, Roger Moore once wrote in Dragon about playing a gnome names Cyragnome de Bergerac, so, that. I've always played gnomes not as 'tinkers' or that other Dragonlance crap, but as the playable wee folk race, similar to brownies and leprechauns. In my mind's eye I can picture a gnome duelist far more readily than a dwarf or halfling.
Elves become a monster in this campaign - my model is the evil Pharisees from Three Hearts and Three Lions, who, incidentally, also appear in Q1 - and half-elves are like changelings, the spawn of the evil elves and humans.
Half-orcs, if they exist at all, are an npc race out there in the hinterlands.
So there, a little monstrous brainstorming to wrap up 1e AD&D swashbuckling & sorcery. If I do end up running this in the future - Las Vegas is calling it 5:4 against at this point - I'll publish a more detailed list of monsters from my campaign.