Okay, first off, I think swarms of anything smaller than rats should be handled as environmental effects rather than creatures. It just seems simpler that way. If you want to stat them as creatures, use the following rules, but give them Physical Immunity instead of Inhuman or Supernatural Toughness (with the same Catch), Diminutive Size, Claws if they're something dangerous like wasps, and Wings if they've got 'em. Give them Zone wide attacks or not as usual based on size. That should give you something appropriate-ish.
Second, Swarms come in two sizes: Person, and Zone. A Person sized Swarm is only big enough to attack a single person at a time, a Zone sized one can attack everyone in a zone. These stats are for Zone sized swarms, to lower them to Person sized, just drop Supernatural Toughness to Inhuman (keeping the same Catch and thus the same cost), and drop Zone Wide Attack entirely.
High Concept: School of Pirranha
Other:
Always Hungry
Can Skeletonize A Cow In Two Minutes
Skills:
Superb: Fists,
Great: Athletics, Intimidate,
Good: Endurance, Stealth,
Fair: Alertness, Discipline, Survival,
Average: Conviction, Might, Presence,
Powers:
Aquatic [-1]
Claws [-1]
Echoes of the Beast [-1]
Inhuman Speed [-2]
Supernatural Toughness [-4]
The Catch is area effects, or anything else that would effect a significant portion of the swarm [+3]
Zone Wide Attack (The creature can attack every individual in a zone with it's normal attack)[-2]
Total Refresh: -8
Stress:
Mental: OOO
Physical: OOOO(OOOO)
Social: OOO
Armor: 2
High Concept: Rat Swarm
Other:
Dangerous Vermin
More Rats Than You Can Shake A Stick At
Skills:
Great: Survival, Fists, Intimidate,
Good: Alertness, Athletics, Endurance,
Fair: Discipline, Presence, Stealth,
Average: Burglary, Conviction, Rapport,
Powers:
Claws [-1]
Echoes of the Beast [-1]
Inhuman Speed [-2]
Supernatural Toughness [-4]
The Catch is area effects, or anything else that would effect a significant portion of the swarm [+3]
Zone Wide Attack (The creature can attack every individual in a zone with it's normal attack)[-2]
Total Refresh: -7
Stress:
Mental: OOO
Physical: OOOO(OOOO)
Social: OOO
Armor: 2
Note: Rats are braver in groups, hence the enhanced mental skills.