You will find that you will crave for a bit more light once your exit pupil below drops below 0.5mm. Therefore 3mm is about you smallest practical focal length. The Nagler zooms are amazing and you will find the 3-6 gets a lot of use for dialling in just the right power for the seeing.
At 3mm you get 168x and this is sufficient for large enough images of planets. If you feel you could push the boundaries a little further you can add a 2 element quality Barlow to the zoom and your total elements will still only be 7 which is the same as all the T6 Naglers but a much smaller apparent and therefore true field.
If using your 'scope without tracking I would suggest the Naglers or Ethos (you never know a 4mm Ethos may be out in the future but 4mm is just a little low in power for the planets on your 'scope but workable). You can get two naglers for one Ethos though.
A 2.5mm or 3.5mm (or both) would be great for your 'scope and maybe complement those two with a 7 & 13.
At 3.5mm you will get 144x which will give you a nice sharp Jupiter and Saturn but you will feel you want a little more image size hence the 2.5mm T6 or back to one or both of the Nagler Zooms. Choices, choices! You could buy a zoom now and once you find the focal length you prefer most of the time then buy a T6 or Radian closest to that focal length for a wider field of view.
You may find you will use your 'scope more for low power viewing which it was really designed for so start thinking about a quality 2" diagonal (if you don't already have one) and your low power sweeping eyepiece.
If your heart is set on the Ethos/Zoom combo then go with it and add more Ethos eyepieces later. If you stay with Naglers consider the 20mm T5 as it is relatively light (2") and will give you 3deg at 25x
Note too the Ethos eyepieces are relatively light considering their size. Have a look at
http://www.televue.com/engine/page.asp?ID=144 for the specs.
Whatever you choose, enjoy!
Congrats with your Sky90, love mine {makes a great little astrograph too}
