The Predalien doesn't make sense because the Alien can't magically generate matter during its growth cycle. A host's health might contribute to the Alien, because a healthy host will provide better nutrition for the developing embryo, but otherwise the host doesn't really contribute anything, except perhaps information.
The Alien takes traits from the host that are relevant to survival. How are mandibles relevant to survival? An Alien does not require them, because it has two arms, two mouths and a highly coordinated tail. Dreadlocks aren't relevant either. Extra size doesn't make sense, since the Alien is already larger than a Predator. Extra strength doesn't make sense, because an Alien is arguably stronger than a Predator already and if not, there's nothing the Predator could contribute to make the Alien stronger.