Well...what about the ssethrics?
The Varn are obviously different. Smite away.
The Black Talon "Fire Dragon" (shhh...don't tell anyone!) IS Kassegore! Their priests use the SAME cants as the Emerald Scale clutch and other followers of the Old Faith .... in that sense the fire Dragon is just a special "ad campaign" or "branding" game. I don't don't know if these Ssethrics can smite each other. I agree this is a gray area. Is the difference between Kas in his aspect as the Great Devourer and his aspect as the Honorable Fire Dragon enough to smite? I personally lean no ... but that's just my take. Would love to know what Henry or even the members of BEAST think.
BUT WAIT! The Ashen Hide Ss'ressen also worship the "Fire Dragon," but they believe the F.D is Nier! And the new version of Coils seems to say that they are now being taught Cancereese cants by Nieirites. Even though they both think they are worshipping the God ... I am almost certain they can and do smite each other.
It would be soooo much easier if your PC would just give up on Jeggal Sagg and do the reasonable thing and start worshipping Tzizhet!
(You would definitely get a cleaner answer of who can smite you!)