Hi Philip,
I also have an 8" F/5 Newtonian as my main scope. My sequence of eyepieces for deep sky observing (i.e. non-planetary) are:
22mm Panoptic
14mm Delos
10mm Pentax XW
8.8mm Explore Scientific
The magnification of the 22mm is a bit low so I'm looking to get something in the 17-19mm range.
I think the range of focal lengths you're suggesting would be very good in your scope, although you might find the 8mm gives a slightly dimmer image than you want. So an alternative sequence might be 17mm, 13mm, 9mm. To reduce cost and weight, I would recommend:
17.3mm Delos
13mm Nagler
9mm Nagler
If eye relief is an issue the Delos are great.
Oh, and try to buy them used on Ice in Space. You'll be able to afford at least one extra eyepiece with the money you save!
Morton