What I did was pacify them during this quest, then talk to them. They will have the same dialogue as they have after killing Harkon on the DG side, then talk to them again (or just skip through their initial dialogue) to get options like "what have you got for sale" or "can you train me in ____?". Then you can interact with them until the spell wears off. To calrify, I was a vamp (stage 4 even) when I did this.
I first thought this might be the case when I pacified Isran right after siding with the Vamps and he offered Two-Handed training. But Sorine, Gunmar and Florentius aren't present in Fort Dawnguard if you side with the Vamps apparently, except during "Destroy the Dawnguard".
I decided to kill them all except Gunmar, since he offers trolls. So I frenzied them all from a dark corner and used Heal Other to keep Gunmar alive. Took a while and there were a few close calls, but Durok finally went down last. Now I won't have to pacify 10 people to get an armored troll.