If the Force operator is evaluated before the FumeFX Follow and the latter has an Influence of 100%, the Force’s influence will be lost - the Speed channel will be overwritten with the Fume’s Velocity.
If you add a Force operator AFTER a FumeFX Follow, it will affect the particles, but as a Velocity and not as Acceleration.
Normally in PFlow, each particle has a Speed channel. When a Force acts on the particle, the Acceleration value of the force is applied to the particle to change its Speed. Over time, a Force will accelerate the particle gradually like in real world, unless another force acts against that (e.g. a Drag Force).
But with a FumeFX Follow set to 100% Influence, that accumulation won’t happen. The Force will be applied on the current integration step, and then the resulting Speed channel will be completely wiped by the FumeFX Velocity. So if you are applying a Gravity, your particles won’t be getting faster moving down, they will be moving down at a constant rate - the Gravity Force turns into a Velocity source instead of an Acceleration source.
If you would set the FumeFX Follow Influence to less than 100%, the Fume Velocity will be blended partially with the existing Speed from the previous frame. But since both the Fume and your other Force will be recorded there, your FumeFX will be blending the current Speed with a fraction of the previous step’s speed, and a fraction of the other Force’s influence. You have to play with the values to see if you can get a better result that way. You can also scale the Velocity by factors greater than 1.0 to get more of the FumeFX influence, but remember that this way a particle might move faster than the fluid that is driving it, and thus produce unexpected (although sometimes fascinating) results.
Hope this helps.