No, but they could magically turn into a serpent, since they are able to as were creatures.
And Atlantean Family Guardians as a dolphin, seal, shark, crocodile/alligator, or other water themed creature would be appropriate and fill the role for many mythologies. Mermaids are just their hybrid form.
Merfolk are definitely a whole separate category, just like Selkie and many other mythos...
I suppose Atlanteans could have their own myths about people turning into sharks, barracudas, squid, etc... Fair enough, I'll have to think more about this.
I'd think that lesser were or serpent (lesser I would see as more appropriate, since it seems to potentially include anything not on the others, which all of the above sea creatures besides snakes falls into) and family guardian would be a better match.
Lesser were actually means "lesser" animals, fox and rats (which are not both rodents, or it would have been were-rodent). I haven't sat down to think of other possible options, but they'll be along that line, assuming it's not more fitting for them to be a Family Guardian (Were-mongoose, for example). My big concern with making Family Guardian open to everyone is that it A) puts in the same category as the Batman aspect. Anyone in the world can be a normal person, those who have a disease, are undead, or mutate/evolve in some way should be more unique, you know?
If they get were-serpent and FG, that also means Hals get FG and have to lose something else...
I think I need to decide on the balanced number first and then work from that...
And in general, especially if you're breaking down weres into so many categories, you could probably further diversify some of the other sections, with equally minor but flavorful differences, for an arbitrary example, sets of witches tied to different elements, with that one thing (picks the flavor and gives a change) defaulted for their abilities, but with unique thematic benefits for each.
I don't want "minor but flavorful differences" to separate the aspects. In your example, when you play a witch and choose your powers, you choose your flavor and your powers change along with it. The were-creatures are being separated because there are significant differences that one type has that another shouldn't. Werewolves can grow to 8-10 feet in hybrid form, a rat can't. Snakes are poisonous, but less likely to transfer their disease, etc.
Although maybe I should go back to having it as contagious/non-contagious Therianthropes with some "standard" separations based on animal... Argh
so many options.