Here’s how the the flamethrower is done in TF2 as a reference.
What’s the estimated lifetime of your clouds? If the player is able to shoot multiple puffs to make a larger persistent cloud, one thing you could do is change the collision spheres on the fly. If there are small puffs that are close enough, their 2 collision spheres can then be combined into 1 bigger one. This can keep changing everytime a player adds a new puff.