While I have made a couple of careers for my setting, they would unfor-
tunately be of little use to you - they are written in German.
However, there are some interesting careers by others over on the
SFRPG board, which have later become a part of Career Book 1, which
can be downloaded from Drivethru RPG, together with a free career
worksheet.
The link to the SFRPG board:
http://www.sfrpg.org.uk/phpBB3/viewtopic.php?f=34&t=177
And there are also some Advanced Education careers over there:
http://www.sfrpg.org.uk/phpBB3/viewtopic.php?f=34&t=455&start=10
And the stuff on Drivethru RPG:
- the Career Book 1
http://rpg.drivethrustuff.com/product_info.php?products_id=58674
- the worksheet
http://rpg.drivethrustuff.com/product_info.php?products_id=59023
There is also a Prisoner career somewhere here on the Mongoose board,
but I do not remember the title of the thread.
As for the balancing, I did not find a way to express it as a rule. I think
you just have to compare a new career to the existing ones and judge
whether it is on the same "power level" and makes sense within your
setting.
In my opinion the only way to really unbalance a career would be to gi-
ve it too valuable mustering out benefits, but this is a matter of taste.