As discussed in great detail earlier, there are several different categories of aliens seen in science fiction literature. This ranges from the utterly silly to the scientifically
plausible. Depending on the campaign, a certain level of Factuality level is called for.
Factuality level is similar to reality level, however since it is pure speculation, the best we can hope for is to gauge realism in terms of facts and educated guesses. That is to say, it is extremely unlikely that "little green men" exist, not because they are little or green, but because they are men. However, it is within the realm of scientific conjecture that me way one day meet up with an alien of an extremely exotic nature, radically different than anything seen on earth.
There is no "factuality dial" per say, rather the factuality level is an abstraction which should be part of the GM's process in developing an alien species. Even if the Gm decides for a "pulp sci-fi" style campaign, there can easily be exceptions within any given universe. If most aliens are humanoid, there can also be xeno-exotic aliens. Likewise, if the universe is populated by bizarre, exotic aliens, there can be cases of localized
panspermia (or "lost colony" scenarios) where there are a few humanoid aliens.
Humanoid, Near-Humans and Demi-Humans
Aliens that look like us! Sure some have elfin ears, spoon-indentations on their foreheads or green blood. Other than some niggling details, they are pretty much human, and more often than not they are sexually compatible. Their culture (though historically different) is easily put into human terms. Their technology, society and even language seem completely human (and easily translatable into English). In many sci-fi shows and books, the aliens even speak perfect English on first contact! Parallel evolution is truly an amazing thing.
Humanoid aliens are the easiest route to take for devising alien civilizations, and some say even a copout. This is the lowest factuality level, at the epitome of space opera and pulp fiction, yet still an extraordinary setting for conflict, epic adventure and
exploration. If humans can commit war and genocide against each other over such minor differences as religion, ethnic race and greed, it is not so implausible to do so with a near-human alien species.
It is, however, possible to explain away the existence of humanoid aliens with the theory of panspermia. It can even be said that a precursor race once populated the galaxy, and we (along with other near-human races) are their descendants (or descendants of the slaves who overthrew them).
For a role-playing game, humanoid aliens are the best choice. Why? Have you ever tried to role-play a two-ton silicate-based methane slug? It is much easier to step into the shoes of an alien who.. well.. has shoes to step into.
Creature-Features
Second in popularity to humanoid aliens, "creature-feature" aliens are quite prominent in science fiction. Many such creatures are hybrids with the humanoid class, that is lizard-men, dog-men, plant-men, and any race with the -men suffix. Other such creature-feature aliens are species that resemble mutated or giant versions of other Earth life, such as Starship Troopers' bugs, giant slugs like Jabba the Hut, and other aliens that make you say "hey that looks like a...". Even if they don't look human, they look like something familiar.
Creature-feature aliens are a compromise between humanoid and exotic aliens.
They are not so exotic that they can't be playable in a role playing game, but yet they are more scientifically plausible (assuming they are done right, avoiding the cliché bugmen).
Such aliens must also arise from panspermia, however, the stellar fertilization occurred at a microscopic level, delivered by comets to a hundred different worlds, rather than the active transplantation of an ancient precursor civilization.
Even creature-feature aliens must have a world, culture, society and even
religions, myths and dreams. Their mentality may be terribly alien, though there could be underlying laws of sentience which help to make certain psychological traits universal;
greed, fear, love, hate, etc.
It Came from Outer Space!
To the uninitiated of science fiction, the first image conjured by the words "space alien" may be a big green monster with big eyes, huge crab claws, two tails and a
drooling tooth-laced mouth. Such space monsters are certainly "creature-feature" aliens, but they should not be considered for a serious hard sci-fi game. Many such monsters are reserved for pulp sci-fi, space opera and movies which can only be watched if shown on MST3K. However, "it came from outer space" monsters do have a place in many sci-fi games.
In a Buck Rogers / Flash Gordon style space adventure, most of the enemy aliens will be space monsters. Indeed, they will be semi-intelligent, perhaps even intelligent, but they do not need to be developed or though out the same way a more logical or "flushed out" alien civilization should be.
The xenomorph from the Aliens series is a contemporary example of an "it came from outer space" alien. However, the popularity of the series, the Alien became a flushed out, well-developed race. Thus, in such a campaign, the heroes might meet up with "just another space monster" only to discover it is the most terrifying nemesis the universe has ever seen.
Aliens of this type are not suited for PCs or even NPCs. They are just space monsters.
Xeno-Exotics
Few and far between in sci-fi are the truly alien aliens. They will have little to no similarity to anything on Earth, and will have utterly alien psychologies. Therefore, they cannot be used for PCs. This is not because they are too powerful or unbalanced, but it is very difficult to play something that things so alien to our own desires. Playing an elf is not too hard, since even elves have the same biological needs and psychological
responses as a human. Even playing an android is possible because that is something a human can identify with. Playing a cloud of sentient neon gas.. well that is just too weird.
Xeno-exotics are best used as a distant alien force. They make good enemies in a space war and are extremely unpredictable due to their alien nature, making it difficult to predict their tactics without knowing their drives.
Xeno-exotics are also good for first encounters and deep space exploration adventures. Finding such an alien and searching them out can make for a colorful story.
Gods from the Stars
The ultimate xeno-exotics in sci-fi are entities of stellar magnitude. Massive energy beings, crystalline lattice entities, sentient planets or stars and aliens from other realities. Such beings are staggeringly old and have god-like intelligence. They are clearly unsuitable for PCs, but make for interesting and dangerous encounters.
Such beings need no stats or skills. The GM simply decides what the being wants to do, and it is done. Simple.