Sources are in ?? /m3/s so it's normal to divide the source by the cell zone volume to get the correct amount.
Species modelling needs a little care, and reading of the manuals & tutorials (Help in Fluent will get you to both). In Fluent we solve for n-1 species, so in your case we'll solve where the SF6 goes and assume anything else is air in the species equations. So, you need to add a mass (total of air+CF6) to the cell zone, and a mass of SF6 (species section).