Not QUITE a zero-sum game:
Survival of the fittest is not necessarily a zero-sum game. There are several factors that can affect your species, allowing for the species to exist in a variety of states. There is increasing evidence that various species of early hominids existed closer together for longer than archaeologist previously thought, so perhaps ...
Smell and rumble
Lizards are known to communicate with smells. Although this is probably not as sophisticated for normal lizards, it has great potential. Smell is incredibly complex in both production and the smelling. If this can be controlled to a great degree, the communication can give both a wide range of emotions as well as complex language. Like sound,...
I mean, I hate to leave what's essentially a one-word answer, but it pretty much speaks for itself. You might say "But telepathy isn't real". Well, neither are dragons. And you'd hardly be the first writer to make telepathic dragons, it's almost a trope, it's so common.
Why Wear Clothing at All?
Consider normal nudity. If these people evolved to live underground, then they've probably grown accustomed to dirt, insects and the like. Clothing is a matter of culture more than anything else, and seeing as humanoids (bigger ones) have historically gotten along just fine & naked (Tierra del Fuego), there's no obvious reason ...
survival of the fittest
It is already mentioned, but not quite to my liking. Survival doesn't mean it needs to be 'better'. It needs to be able to reproduce in a (semi) sustainable way while successfully competing for resources.
In our world we can be seen as the apex organism and there's nothing better than us. Yet we're surrounded by other life. Plants ...
The only fibers, or fiber like structures, which they can access underground are roots.
The roots of adequate diameter can be dried and woven together to produce a very rough fabric, at most resembling yuta bags, which can offer some protection against contact with the underground. This is what they probably actually need, since a few meters underground ...
Lycra / Spandex swimsuit.
These are a real thing and can be made to whatever animal form you want. The closest I can find to your picture is a horse, but shouldn't be a problem to someone with a sewing machine and a few bolts of spandex.
In almost all cases of elves and humans co-existing, with elves having these archetypical long-life traits, elves live separate lives.
Wood elves live out in the wild woods, i.e. where there is little in the way of human style civilization.
Dark elves live in caves or other areas inhospitable to agricultural human life
High elves tend to ...
Your ecosystem needs fleshing out
Using radioactivity to power an ecosystem is iffy - it's hard to suppose the amount of radioactivity the genetic code can withstand will be enough to power an organism directly. One option is to use an intermediate chemical, such as hydrogen gas produced by radiolysis of water, so that radiation over a vast subterranean ...
The archetypal Western dragon is naturally able to breathe fire, so a useful byproduct in this case is smoke. The same muscles that enable the dragon to control its propulsion of fire(or a flaming something) out of its body would also allow it to control the attributes of the resultant smoke. It may prove inconvenient to have to incinerate ...
Dragons are often portrayed with frills, spines, jowls, and other protrusions on their heads for no readily apparent reason. Many animals use their headgear - ears, whiskers, and the like - to communicate; cats and dogs are well-known for their expressive ears, for instance. (Of course, cats and dogs are also known for using their tails. ...
They don't need to.
Larger animals need more food. The larger a species, the lower its population density can be based on resources. This is especially true for predators, since around 90 percent of the energy is lost at each step in a food chain. The population of a large predatory species such as a dragon will necessarily be much smaller than the ...
Infrasound transmitted via the ground, using their feet as sender/receiver.
This additionally allows them to have an overview of their surrounding without having to rely on sight.
If I remember correctly, elephants use something similar.
Its more a question of how the Elves managed to survive than the humans.
For example division of labor. For each scientist you need people that sustain them, from food and water production to industrial capacity to build the things they need to creating the luxury products and free time that help the scientists do their job.
The Elves might live longer, but ...
As the question already mentioned, delayed maturation and limited propagation rates can be used to offset the competitive advantage provided by extended lifespans.
There is an all-too-short class lecture in the movie, Lucy, which contrasts these two survival options and the environments where each of them thrives.
Its conclusion, that harsh environments ...
Use a poncho, with extra buttons on the bottom.
Or a sack, with four holes - five counting the tail - and a very wide mouth; the mouth can be closed by pulling a string, all around the neck.
The poncho would probably work better when exiting the water, as the soaked cloth would otherwise drag underneath.
Odours - Consequences
This was meant a s comment to @Trioxidane answer but go too long:
Odours as the primary form of communication form has several "disadvantages":
Producing different pheromones/hormones is hard and need genetic requirements. Even then, it's hard to convey complex information. I'd say to estimate how many odours they'd need to ...
Dragons tend to be pretty solitary animals. They rarely have a need to communicate with each other aside from to discuss territorial disputes and to impress mates. In both of these situations being having enough energy to preform complex dance moves is beneficial both to intimidate and impress.
It isn’t exactly a language however it doesn’t have ...
Could they exist? Yes.
Would someone tall and lanky, like me, want to live in a cave where I’m forever banging my head and slouching? Never. I would hope evolution steps in to make creatures like most other subterranean animals; horizontal (i.e. naked mole rats, moles, worms, voles, etc.)
If you can't beat them, be them.
Cro-magnon man did not ethnically cleanse the neanderthals. Cromagnon man assimilated the neanderthals. One human group showed up and interbred with the people they found. They are both our ancestors. Humans breed.
So too the demihumans. Humans can breed with all of them and there are a lot of humans. Human genes enter ...
You can have an elf race breed like humans and mature normally and still be at a disadvantage. Immortality isn't always a benefit, for several reasons.
The carrying capacity of locations is fixed.
Each location can produce a certain amount of crops, has a certain amount of water, and has fixed resources. Humanity can push up against the carrying point just ...
Reason for Elves' stability with humans
In most media elf fertility is extremely low.
The Brain, an elf brain most likely requires more development time (childhood) than humans.
Civil Discord and Sub-races, think dark elves and high elves. Dark elves aren't just black elves, usually, they evolved into a different species with different magic.
War, if an elf ...
Fins just need to be seen as sexy
There are many reasons for evolutionary features to evolve other than pragmatic ones.
For instance something could evolve because it is sexy. As your merfolk culture is half human, one could expect a similar emphasis when choosing a partner to be on traits that are sexually desirable, perhaps there is great respect for other ...
Your merfolk do not use hands to swim.
Let us consider the Man from Atlantis. The 1977 TV one starring Patrick Duffy, who went on to have some fame as Scuzzlebutt's leg.
He swims using his back and legs in a butterfly kick. His hands stay at his sides, sculling. He never reaches forward. Here is a good gif of ...