The spores are going to need four properties:
- Bioluminescence (check)
- Highly reflective
- Gliding ability
- Attraction to the swarm
In addition to providing their own light, to get a flickery, fire-looking effect you want them to also be reflecting the light of the other spores. Instead of being a simple static glow, having each spore sport a number of shifting reflective planes will give you a much closer approximation to a floating ball of flickering flame.
The spores are going to be too small and mechanically simple to properly fly. But! Their light weight and some well designed fixed wings could give them the ability to glide for quite some time. This will help keep them up in the air as a "will-o'-wisp".
Attraction to the swarm
A quickly dispersed cloud of spores is, frankly, useless. Like throwing shiny dust in a high wind. We want a coherent mass. And that's why we need an attractive force. Not too attractive, or we'll just have a big ball of spores that falls directly to the ground. Which means we have to sporadically generate static electricity. The random static pulses will pull the swarm together briefly, then it will all begin to drift apart again. This will repeat as the swarm slowly drifts to the ground. The fox fire will collapse and become a bright point, then disperse and become a faint misty glow.