According to Wikipedia, Musteval's are not rat-like, but ferret-like Guardinals (which I had not heard of until I looked them up). Ferrets are not rats, and in fact eat them and other such pests. They are kept as pets in part because they catch rats like cats do, and have also been used for Rabbit-hunting for thousands of years. Wikipedia also says that Mustevals serve mostly as spies and messengers, which sounds just about right for the recon line.
I don't think I could be convinced to use rat-people, but I had actually thought that ferret-men could be kind of cool when the rat-men were first introduced. These would be good for the scouts and assassins, but I think I still prefer Dryads for Druids. It might be odd though for a ferret-man to upgrade to a nymph.
(In my modmod I'm thinking that I'll probably make Druids available to all alignments, plus have both nature ans life spheres. They would however require a minimum level, to be upgraded from recon units or FoL disciples. I might make Dryads a UU that is the same but has the Elemental (or maybe angel?) promotion and no level requirements and to which no unit can upgrade.)