So are those like a user name and password for controlling Daedra?

A fair few creatures in real-world mythology can be summoned, banished, or controlled at threat of banishment via use of their name, pre-victorian fairies (the malevolent kind) being a good example. This is a fairly simple extension of that, with the difference being that Daedra have an "original name" that never changes (the protonymic), and a "new name" that is given to them every time they return to a corporeal body from the waters of oblivion (the neonymic) to make it much harder to banish them using just their name.

....so basically, it's a user name and password to get administrative control over them?

No. Read the post.

Knowing the neonymic and protonymic to control a daedra could probably be considered similar to blackmail. The daedra isn't forced to obey you, but not doing so could be detrimental to their wellbeing. It being "like a username and password" would imply you had absolute, unlimited control over their actions, which is simply not true.

You need both. Knowing the Protonymic won't do you any good without the Neonymic. "http://www.imperial-library.info/content/havok-wellhead"

Does anyone know is the ~nymics are limited to only Daedra?

Whatever you do, http://www.imperial-library.info/content/skyrim-tragedy-black.