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YES. a 17mm eyepiece in a 6" DOB will allow you to find the planet and centre it. You should get a clear (albeit small) image.
Then you increase magnification, either by adding a barlow (say 2x) or a lower number eyepiece.
I was looking at Jupiter, Saturn and Mars last night.
I am at a reasonably dark location, but not totally ideal.
I have a 10" DOB (Focal Length 1200)
The images were all fuzzy.
I was using a 12mm (TV Radian) with a 2x Barlow(Orion shorty-plus).
Yes you need to re-focus (generally) when changing eyepieces or adding a barlow.
Best advice is to focus on a near by star, until the image is as small as possible. (ie turning the focus nob further results in de-focusing the star and the image bets bigger and blurrier). [For Jupiter focus on a close moon].
At that point the scope is focused and you can then concentrate on the planet.
For me, that gave a magnification of 200 (Being1200/12 )*2
The image was wishy washy and not at all sharp. hard to make out detail.
Next I tried a 9mm (TV Nagler). - NO Barlow.
This gave me 133 magnification. (Being 1200/9)
The images were pretty small, and still not really sharp.
You could just make out the bands of Jupiter, but a novice I was with could not, nor could she determine the Cassini division in the rings of Saturn.
What I am getting at is that maybe you are expecting too much from your scope, especially where the atmospherics are far from ideal.
A week ago, at our Dark Site, the images using the same equipment were razor sharp and detail at 200x was rather good. (But not for Mars which is experiencing a planet wide dust storm which makes it look like a blurry featureless ball.
Sorry for rambling, but I hope relating these experiences helps you to understand the limits of planetary viewing.
I think your scopes Focal length is the same (1200)
So your magnifications would be similar for similar strength eyepieces as I mention.
I suspect your best planetary viewing would be at around 150 times (ie an 8mm Eyepiece or 16 with x2barlow),Plus or Minus!
Regards
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