I separate like colors together. Depending on the color, I'll wash in warm or hot (hot for heavy bleeders). I do use detergent, but not as much as I would use for laundry. Enough to get the processing chemicals and sizing out. I use a delicate cycle. I dry in a warm dryer, take out when dry and fold immediately. I don't iron or starch until I'm ready to cut.
Why would you want to wash only "most" of them and not "all" of them? If you some and not others, the ones that were not pre-washed will shrink while the others will not. If your bargello uses narrow strips (1" or less) you will really notice that after it's pieced and quilted.