As I thought, It is probably impossible to combine additive and blended particles in one off-screen target using only one pixel buffer, cose thay use different method of background modification (it cannot be accumulated only in buffer - cose strenght of additive effects impact on off-screen target depends also on the background).
However they cannot be rendered separately, because they have to be sorted.
Using both blended and additive particles is common in games. We have not tried this, but we believe that it would not be possible to combine both in a single off-screen buffer. Supporting both might require a separate off-screen pass for each.
For example fire would make particles brigter (maybe this impact would be hidden in dense mist of particles, but impact on surroundings would be lost). However it is a nice quiz, I think it can be solved with some masque of "influence on background" and "infuence on background brightness modifier" (based on transparency) that will be modified in real-time.
After putting depth test shader to particles material, I'll put it in material scheme listener, and start tidying up code.