from what i can remember the alternate types
1. Doppelgangers: basically an identical copy to the person they've mimics
2. Flawed Impersonators, aka Overdriven Assimilators. they're flawed, their features are extremely exaggerated (or "overdriven") to the point of body horror/uncanny valley such as elongated limbs, upside down heads, multiple eyes, etc
3. Unspeakables, previously known as "Detectable". only thing i remember is that they're basically immortal/overpowered and that Alt!Gabriel was that species
4. Tulpas. willed into existence by the subconscious of children, like imaginary friends