So I can choose Human hybrid with Lucky and gnomish Fadeaway, who can speak with animals, is always well feed and rides a giant green furred hamster and lives for 400 years? Cool
Yup. This character concept is fine. As a DM, I would love a character like this in play.
Relatedly. Tentatively for now, the Original Post only allows a player to pick the traits from two Species for a Mixed-Species character concept. The reason for this caution is the "possibility" that unexpected combos might be more powerful than the design intends. Strictly speaking, the more things to choose from the more a combo might happen. But I doubt there is a problem here. Because, 5e players have been combining feats for almost ten years, and no feat combos have broken the game so far. It seems lumping the Species traits into special feats will continue to be robust. So maybe later, the restriction to only choose two Species for traits might open up to allow multiple Species to choose from.
Assume the limitation of two Species remains cautiously in effect. And assume Halfling remains a separate non-human Species. Halfling Luck is not powerful. It is great flavor but nothing to worry about balancewise.
So to build this concept, this Mixed-Species character can have ancestors from multiple Species. Suppose the mother of the character appears Human: her father is Human but actually her mother is Halfling. Then the father of the character is Gnome. (But who knows, perhaps the Gnome has a great-great-grandfather who is a Dragon!) So the character has three Species who can transmit traits: Human, Halfling, and Gnome. To build the character concept, make Gnome and Halfling the two Species whose Traits are being expressed.
• Gnome Base Traits: Humanoid, Small (4 feet tall), Speed 30, Lifespan 400
• Gnome Fadeaway
• Gnome Guardian of Animals (Speak with Animals, Comb, Feed, Animal Rider)
• Halfling Nimble (Luck, Nimbleness)
This is a fun concept to play.