Most of the JG fittings HBS' stock are for 8mm OD line
Also the cheaper 6pm ID line that you can get is softer and doesn't seem to work so well. My advice would be to use the 8/5 mm Valpar FlexMaster II line
Places that sell water filters will have the 6mm 1/4 inch JG fittings and you can get the right size tube from them as well.
Quick release air compressor fittings while cheap and readily available are generally not suitable since they leak costing you a cylinder fill and flat beer.