In terms of stats, I'd suggest something like the following:
Move – 7"
Wounds – 5
Armour – 1 Weak
Magic – 1
Morale – 8 Fearless
Melee – 5/2 weak
This is just off the top of my head for a generic swarm – movement to suit the swarm type, multiple wounds, low defences, multiple attacks. The 'Fearless' rules seem to fit swarms well, you may consider 'Berserk' for certain swarms (rats, for example).
Jungle swarms could have higher strength to represent poisoned attacks.
Bat swarms would obviously have faster flying movement.
Ghosts would be a bit difficult – perhaps a fixed save and low magic defences (so it's hard to kill them with 'normal' attacks, but magic hurts them).
This models them in a similar way to Warhammer (the swarm gets all its attacks or none), which I think is fine. The alternative would be a costing system similar to the heads of a hydra.
As for deployment, any unit (other than berserk units and possibly flyers) can be in rank & file or skirmish and I don't see why swarms should be very different.
