The FumeFx stuff still requires the ‘Smoke’ channel, which this is directly translated into Krakatoa’s ‘Density’ channel. You are correct though, that the ‘Fire’ channel could be used with a Density of zero, so I’ll look into that eventually. Right now Fume support still hasn’t been confirmed by Kresimir, so I’m reluctant to polish it further until I know if we are allowed to give it to you 
In additive mode, no lights are required because the particles are made emissive(ie. emitting their own light from the ‘Emission’ channel), and have zero Density so that they don’t cast shadows on other particles. Hence, it does not require any lights.
I didn’t allow Fire without the Smoke channel, but we did sort out a couple of things Fume-related. Most importantly, we have inked an agreement with Kreso to officially allow Krakatoa to support FumeFx.
Rendering with Fume should play out like this now:
- The Smoke channel from Fume is translated into the Density channel of Krakatoa.
- The Velocity, TextureCoord (FluidMapping), Temperature, and Fuel channels are all available to Krakatoa Channel Modifiers.
- The Fire channel from Fume is directly available in Krakatoa Channel Modifiers, assuming that the simulation has saved the Fuel channel.
- There is a Krakatoa FumeFx modifier that can be placed on FumeFx objects in order to disable the loading of specific channels, or the entire FumeFx object. This can be used to disable specific Fume objects in the scene, or to prevent costly channels such as the Velocity channel from being loaded by Krakatoa.
I’ll make a note to look into allowing Fire where there is no Smoke. Let me know if anything is not working for you, or if I am missing some important features.