Do flamer weapons even have the chance of lighting hordes on fire? Or do they just do the magnitude damage and that is it

Just the Magnitude loss.

by the rules it makes only magnitude loss but you can still make a home rule.

Btw I've change heavy flamer damage to 3d10 instead of 2d10.

When shooting at a horde with a weapon that got the flame trait make (range/3)+1d5 magnitude loss (as usual)

Then make a BS roll for each degreee of success make an agility roll for the targets if fail it caught fire but still alive run madly around the field of battle.

Duration:

1-10 wound ennemy run for this turn only

11-20 wounds ennemy run for this turn and the next one

21- 35 wounds ennemy run for 3 turns including this one

...

Each turn they make an agility roll at -20 if succed the fire is extinguished and the ennemy die with a delicious barbecue smell (must to love burning prometheum smell indeed), if failed roll 1d5 agility roll, each roll failed cause an other ennemy to take fire...etc

For every 5 ennemy in flame reduce the horde magnitude by one.