What CuSO4 wrote is correct.

Some additions
| 
| > Has anyone ever hired seiyuu from a single management 
| > agency for any single production or project?

For many of the TV anime, a few of the major characters are
gotten via auditions or so, but the rest of the characters and
guest characters are usually gotten from a single production
(usually the same production as one of the major characters).

In the ending credits, at the end of the seiyuu credits,
you can see something like "kyouryoku so-and-so production".
This means that the director just let the production
worry about getting the minor seiyuu for the minor roles.

| I thnk the Kira Kira Melody game was using all the seiyuu
| of a single managemetn agency.  It was its intend, but I 
| do not know if the game was finally released or not.

Kira Melo was a project to raise seiyuu, but a few of the
girls had already belonged to certain productions, so it wasn't
100% Mras (the seiyuu production working with Kira Melo).

With games it's much easier than anime to use seiyuu from
just one production, as there aren't many characters and
you don't have to worry about new guest/minor characters
every episode.

--
Hitoshi Doi                                      "this place also for rent"
                           " this space for rent "
http://www.usagi.org/doi/                                     doi@usagi.org