I just realized that there's a problem with mob were-swarms.
And as I'm sure you remember, swarms have the same size as their individual member organisms. A viper swarm is Tiny, but a kobold mob is Gargantuan.Originally Posted by DMG 2, page 59
That said, by RAW it's still possible to have a mermaid mob werewhale—that is, a bunch of mermaids who Voltron into a whale. Elephants, rocs, giant snakes/squids/sharks, some dire animals, and dinosaurs are also valid, as are some weird things like sea tigers and bilge eels.