A Druid makes sense, but a Cleric using Blight Magic is even better. Especially to a god of Pestilence and Poison.
Be CAREFUL with that carrion crawler. I nearly wiped out a 7th level party with 3 carrion crawlers (EL 7). It gets 8 attacks (even if they are a +3), so that will at LEAST be 3 fort saves they have to make in a row.
If you can, have the Cleric use some blighted undead. The Ranger will HATE them. Have some come out of the water, perhaps? Grapple PCs and pull them in?
Maybe the Ratmen could be spreading disease by animating the corpses of animals (Snakes, mice, etc), and using them to move through the food in the marketplace, spreading disease.
A medium giant spider should be a good chalange too. Or several Small ones.
Maybe the PCs could meet a Ghoul who doesn't immediately attack them. Maybe he doesn't like the ratmen, and can give the PCs information... if they help him. Bring him food. Or the Ghoul could be working for the Ratmen. Lurking in the water, a look out, he swims back, tells what he sees. Or a Ghast, that uses his stink as an alarm system.