I'm picturing the Ba'ku people from Star Trek Insurrection,

More or less. A little less, perhaps. Perhaps more like Native Americans to some degree.

Perhaps in describing the tumans, I put too much emphasis on introversion. In terms of loving relationships, friendships, and so on, they keep very close-knit, but they do have larger societies, they just treat others with stoic reserve until they get to know them better.

As far as fables goes, I might consider something like that, though I think culture-wise, the bumans are more into that kind of thing, since they like to tell stories, and, I think, have a grander sense of hero-worship than the other races. Lumans are more prone to make up fictional stories, while I think tumans are more down-to-earth and present-oriented. But Of course I may change things up as the world evolves; I recently made a massive change to the physiology of bumans after I thought I had that locked down, so we'll see.

If they live on separate homeworlds, how do they meet?

This one takes a bit of explaining, because the physics of these worlds is counter to our universe. I'm thinking of posting something like "Worldbuilding 101: What is the Gestalt of your world?" and give this as an example:

Worlds do not orbit stars, but rather float through a lilac-colored cosmic aether along hard-to-predict paths. Miniature suns orbit each world, though this number can be different per world. Bumans have no suns at all, and live in a dark, snowy world; tumans have three suns, and have a hot, humid environment of archipelagos and deltas; lumans have too many suns to count, baking their world into a desert.

So these worlds (and infinitely more) slide past each other, coming close and moving about the heavens in a grand cosmic dance.

Now that you know how the universe operates, you see that such worlds can come in fairly close when they pass.

But how does one hop from one world to the next? By waterrises. Like waterfalls, but they rise up into the sky, defying gravity. If you want to visit a world, you must track where the worlds will be passing, swim down a river, and dive up the waterrise, aiming for another world. But, of course, don't forget your parachute, or you're in for a lethal landing. (Parachutes, of course, invented by luman clothiers.)

This particular aether-and-waterrise system was developed for a number of reasons in regards to gameplay, but in terms of setting, not only does it look cool, but it allows for a lot of variety of biomes and worlds that can be reached quickly without too much grand traversing and long miles over similar terrain. But also, since the dance of the worlds is difficult to predict, only when worlds get close can people really figure out where worlds will be connecting. You might also be stuck on any given world for weeks or even years until the worlds align for you to move on. This can also create a disparity of technology, where one world might have so many resources available that they can create grand cities, while other worlds lacking in resources are capped out at some level of tech. Since traveling by waterrise is rather bumpy, it's hard to bring too many supplies to a resource-scarce world (and besides, you wouldn't want to be dependent on imports because you'd never know when the next time would be that someone could bring you some).