I have a CD of the Crimean War Medal Rolls wherein the 6th Dragoon Guards is conspiculously absent from the 'bookmarked' regiments. Nonetheless the 6th Dragoon Guards (Carabiniers) did serve in the Cavalry Division of the Crimean War. Ancestry.com has also omitted the 6th Dragoon Guards from its medal roll index.
I have discovered that the Carabiniers records in WO100-24 have been tacked on the end of the 5th Dragoon Guards medal roll records and begin on folio 108 and end on folio 118. I imagine they are not complete.
By careful cross-matching of a selection of names, I have discovered that ancestry.com has indexed the men of the 6th Dragoon Guards as 5th Dragoon Guards.
Hope this is helpful to those who have not yet discovered the same problem.