I am designing new business cards for employees and many of them have several credentials after their name. To make them all "fit" do I bring the entire name and credential line down a point or can I just make the credentials a point smaller? Is there a rule to this?
Yes, go ahead and make it smaller. I second @Scott, most cards I have made have the credentials smaller (though I come from a country where little importance and attention are paid to credentials..)
Should you decide to make it smaller, I would suggest actually going possibly a couple of points down. If it is the same fontface, a too-small difference in size might make it feel a little "off".
Italic generally uses less space, and personally I am not a big fan of small caps.
From a Design point of view, I agree that dropping the font size on the credentials makes sense.
Just make sure your client agrees...
Many people work very hard to earn that string of characters and it is very important to them. To those clients, minimizing the impact of their efforts can be as insulting as spelling their name wrong.