From the factory configurations...
...the Kel-Tec SU-6A, SU-16B, and SU-16CA are CA legal.
... the Kel-Tec SU-16C, SU-16D, and SU-16E are not CA legal.
Possible complications, because the SU-16CA has a threaded barrel...
^If a flash suppressor/hider is installed on it, then the SU-16CA needs to operate using a fixed 10 round magazine (maglock + 10 round magazine).
^If a muzzle brake or a thread protector is installed on it, then it is CA legal and no mods needs to be done to the SU-16CA.
In order to be CA legal...
^The Kel-Tec SU-16C needs to have an overall length of 30" or greater with the stock folded and needs to operate using a fixed 10 round magazine (maglock + 10 round magazine).
^The Kel-Tec SU-16D is a SBR, which makes it too much trouble to deal with to make CA legal.
^ The Ke-Tec SU-16E needs to have an overall length of 30" or greater with the stock collapsed and needs to operate using a fixed 10 round magazine (maglock + 10 round magazine).