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The 21E has a 36.2mm field stop diameter which is almost a perfect match for the 8" SCT's and will provide you with a 1 deg true field at 97x and 2.1mm exit pupil. (2032mm f.l.). This would be one very nice DSO eyepiece with plenty of true field and magnification. The classic alternative is the 35mm Panoptic with 38.7mm FSD and 1.1deg true field at 58x and 3.5mm exit pupil.
For planets either the 13mm for 156x, 10mm for 203x or 8mm for 254x.
The 21, 13, 8 would make a nice set but note on planets you may want something between the 8 & 13.
You could also go with the 17 & 10E. The 6E may be too much power most of the time. The 17E with 29.6mm FSD will give you 0.83deg at 119x and 1.7mm exit pupil. This would be fantastic for a full lunar disk at 119x and also good for the larger planets.
You know you will end up with more than one so toss a coin between the 21/13 combo or 17/10 combo. If you want the wider true field and do not have another eyepiece with a wide field stop diameter then go the 21E unless you plan on getting the 35 Pan or 31 Nagler later.
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