RAW being one creature I would think the swarm can't be cleaved.
A swarm is immune to spells that affect a number of creatures, even haste which can affect up to 10 easily or 20 theoretically. This would imply that cleave hitting 2 or 3 or 4 people isn't an AOE it's just a single attack.
Even if you're a dragon kin with a scythe for a pole arm, against individual creatures you're going to miss before you hit the theoretical 100ish creatures you could have in your reach.
As an ad hoc thing, maybe bump cleave up to 75% damage and great cleave up to 100 percent damage against troops?