Races in Cardinia all share a common ancestor and are capable of procreating with greater ease than in other settings. Races are divided into three groups: Mammalian, Oviparous (egg laying), and Magical. Races that belong to one group can reproduce with other races from the same group (but not differing groups). The only exception to this rule are creatures belonging to the magical group.
Bugbear, Bullywugs, Centaur, Changling, Deep Gnomes, Duergar, Dwarves, Elves, Firbolgs, Gnomes, Goblin, Goliaths, Halflings, Hobgoblin, Humans, Minotaur, Myconids, Orcs, Shifter, Tabaxi
Aarakocra, Dragonborn, Harpies, Kobolds, Kenku, Lizardfolk, Naga, Tortle, Tritons, Yuan-ti
Aasimar, Gensai, Tieflings, Warforged
Races from the magical group do not procreate and may be combined with any other single race. Human/Gensai is fine, but there are no Half Elf/Tieflings or Wood Orc/Aasimar.
The process for combining races varies based on the parents. If both parent races have subrace options the process is straightforward: take the base race of one, and the subrace of the other.
If neither parent race has a subrace the process is also easy: take the ability score increases, size, age, speed, vision enhancements (or lack thereof), and languages from one and the remaining features from the other. The only exception is humans. Instead of the normal ability score increases granted to humans they instead gain +1 to any two stats, and proficiency in one skill.
If only one parent race has a subrace the process is more complicated and must be handled as a case by case.