Dj, the Shuffle mode is the 5.5G, with or withour Rockbox. No one knows yet how the Solo will work with Classic using Rockbox. Ken is on record as saying that it won't work but he hates Rockbox. (In fairness, it gave him nothing but grief with the iMod... warranty complaints that were Rockbox, not hardware related).
I think that it just may work. However, there may be further reduction of battery life. Anyway I would wait until at least the beta release on the official Rockbox site. This pre-release could brick your Classic.
Edit: It has now advanced to "unusable port" status on the Rockbox site. Once they solve Dual Boot OF and Install with Rbutil, it should be okay.
Edited by cooperpwc - 4/20/11 at 9:57pm