Kimberly, I don't know if you've worked this out by now (hope so!

) but I can tell you from experience that getting custom designed newsletters to render properly across all email clients is a bit like getting a website to render across all browsers. It just takes some tweaking of the code until the design works well in all email clients.
We always check in Yahoo!, Hotmail, GMail, Outlook... etc. etc. and invariably will find issues with one or two (usually Yahoo and GMail).
The good news is, once you solve the issue you can reuse the same template and won't usually run into future issues.
Good luck!