I just realized that there's a problem with mob were-swarms.
Quote Originally Posted by DMG 2, page 59
Size and Type: A mob is a Gargantuan creature composed of either forty-eight Small or Medium creatures or twelve Large creatures. The mob’s type remains unchanged from the base creature.
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.


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.