The 9mm will be your workhorse eyepiece for detail in that scope as the seeing will limit the magnification most nights. Last night I was observing the moon and my 9.7mm gave good views but the seeing was not enough for my 7.5mm.
A good 2x barlow would be a good investment and would cover you for most nights. Just remember that a clear, small view is much better than a fuzzy, larger view
