General rule from one eyepiece to the next, choose a factor between 1.4X and 2X on focal length. I'd say skip the 6.7 unless you plan on buying the full set - go straight to the 4.7 mm, which will be close to the limit on magnification.
For a dob a set of 3-4 eyepieces is enough to span the useful magnification range.
Resist the temptation to buy a suitcase full of big fat lumps of glass. They are unnecessarily heavy, big and fat.
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