Imo you cannot compare the gate and harpy with sersiryx because of the different wording.
Sersiryx says 'only fire' and the trident is fire AND dark. This means when your casting the trident your casting a dark spell, though your also casting a fire spell.
If that's a clear cut 'of course he can' to you, then fine. Its definitely not cut in stone for me. I can see this go either way, thought I believe with the current wording its a no.