Life as we know it is built out of DNA. DNA is composed of four nucleic acids. They pair up and construct short sections of three to grow into everything we know. Humans, rabbits, star fish, hummingbirds, spiders, all the same tiny pieces arranged differently. Guanine, Adenine, Thymine, Cytosine all appear in amounts that are tied to the one they pair with

If you start of with the basic assumption that you are keeping all the rules for how earth biology works except for a different number of base pairs and base items, you’re gaining enormous flexibility. Realistically, you could see as much variety in a population of 1000 adult alien sentient critters from the same large town as there is in every species of canine on the planet. Colors, shapes, sizes, number of claws, dominant gene sets, temperament and intelligence

If your Notionalbiology had two more base parings? In earth life A and T pair up as do C and G. Say, for example there Xamplinine and Notionsine which were another pairing, as well as Handwavesine and Fakenine. Scientifically they’d get imaginative shorthand like X, N, H, and F. Given the variety of life we know of, you could have a planet spanning species with huge regional differences in phenotype expression, and yet still be inter-fertile. We know that wolves in northern and colder climates will be larger than ones in southern and warmer climates.

With this much biological variability you could easily have subterranean specialist, water dwellers who spend their whole life in the ocean, and some gifted with flight all of whom could produce offspring of the same species within their own group or as crosses with other groups.

Strictly speaking, as a purely biological phenomenon, if this species were aggressive, they’d make the Borg look like the pushy kid at the local lemonade stand because in a single generation they could adapt to just about any environment.

Now some links from people who know what they are talking about: