I second mszodiac about the SDS issues.
Although I have had different releases of the same version in clusters without too many issues. For instance 6.0 SP2 and 6.0 SP3 sync ok in the same cluster. However, I wouldn't do this with 6.0 SP3 and 7.0.
However, even this isn't without some problems. For instance, the latest version of MiCollab requires a specific release of 6.0 SP3 and above.