Are there some special rules about fighting against swarms (a bee colony for example)?
The magic system knows them (Target: group), but what about fighters?
I'm not sure if Ars has rules or not, however adapting the basic concept from DeathWatch rpg might work.
From memory (probably inaccurate ) Basically damage which hits inflicts a reduction in the swarm's size attribute rather than normal damage. When the swarm has no more size it's defeated. Each body level equivalent reduces size. Power/items such as jets of flame should have an increased effect after the stamina soak.
And each round the swarm takes large damage it needs to pass a cohesion check, but I can't recall the mechanics of that.
Thrice Told Tales has some stuff (including rules) on swarms and how to fight them. Page 120.
Perfect! Thank you very much!
Guardians of the Forest has some rules for mouse swarms and thus predates TTT. I hope the same rules are used
The rules are quite different... But in GotF we have infernal mice with a swarm power, in Ttt we have mundane spiders and swarm rules. Maybe that´s enough to justify a difference.
Couldn't you have mentioned this during playtesting 
NDA! I can't remember, maybe I did?
Anyway TTT has rules for having swarms of small creatures or "worms" with a total Size of +1 or more fight as normal with the well known stats. But they have a few differences regarding ignoring Defenders and reforming as a new swarm after being damaged. Plus there are ideas about having non standard actions in combat be effective, like swatting with a broom.
In GotF rules for mouse swarms (size 0) define the adds to a damage die depending on armour coverage. In combat the Brawl total reduces the number of mice and after a given amount of losses the swarm breaks up. No normal combat, soak and the like.
I think the older rules are nice and simple and the newer ones fit the standard rules for combat better.
Legends of Hermes had a box insert on a scorpion swarm on page 44.
I just compared the swarm rules in Thrice Told Tales with the swarm rules in Legends of Hermes. They are nearly the same, although the rules in Thrice Told Tales are somewhat more detailed. Even the phrasing is predominantly the same. So, I think, the state-of-the-art swarm rules are contained in Thrice Told Tales.
(I need them now for the bats in "bats of mercille").