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[bogus@does.not.exist.com ()]

"The base SL distribution is basically Enterprise Linux, recompiled
from source.

Our main goal for the base distribution is to have everything
compatible with Enterprise, with only a few minor additions or
changes. Examples of items that were added are Alpine, and OpenAFS."

I'm not sure what you mean by mixing EL packages with SL packages--why
would you mix RHEL, CentOS, and SL packages on the same system?  Or do
you mean third party software?

CentOS and SL are both designed to be compatible with RHEL.  And they
both do a pretty good job, but there can always be unexpected issues.
See this one on CentOS for example:

http://lists.centos.org/pipermail/centos/2011-November/119250.html