Designing an Online RPG

Would you develop all the classes and test them, or test each one immediately after developing it?

you'll notice that most mmorpg's do not make every conceivable class at once

they develop a small basis to begin the game with while the community grows, and add more content / classes as the community grows larger or reaches the limits of the existing classes

It would honestly depend on the class.

Each class has it's own abilities/weapons.

You wouldn't just be testing the class itself, but also the many abilities and items that go with it.

Probably make them in balanced pairs, per following example.

Start with a Fighter class (i.e. high attack power, but low magic) and a Mage class (low attack, high magic) and test them against enemies with corresponding stats (high defense, low magic defense for the fighter; etc.). Then repeat.

Actually more liek against a variety of enemies, as well as probably people playing as the other class if PvP matches are to exist just to balance them out. Hell, do it even if you don't plan on including PvP just for ease of balancing them.