I've run a version of MEPIS as my main OS since late 2003.
You can enable all the repositories that are there except the deb-src one and Virtualbox without problems. You need the multimedia one for some codecs that are not free.
I'd recommend you have things set to Prefer versions from mepis. Check this setting each time you open Synaptic as some of us have had it change on its own.
Try our Wiki for info on software and installing it. Here
http://www.mepis.org/docs/en/index.php/Main_Page
You should have a link in the Firefox that comes with MEPIS.
As an aside how can the Ububtu repositories have more programs that Debian when Ubuntu is based on Debian?
As for Stable etc.
Stable=Stable (it will have no problems. period)
Testing or Lenny=(This software is basically a release candidate)
Unstable or Sid=Code is changing and may (probably will) cause problems
(beta software)
Experimental=Alpha software
Many people use Sid for the newer versions of software. If you do you get to keep the pieces when things break.
Testing is safer but can still mess things up, just less often. It's called testing for a reason.
There is a wealth of information at
www.mepislovers.org come join us.