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A 10-12mm eyepiece is your best bet for maximum detail. 7mm in your f/5 will be getting too dim and with the 20mm the magnification is not enough. Does not have to be an expensive EP, but helps to have a well made one, with good multicoatings. But probably even your 20mm in a 2x barlow would not be a bad start.
You'll also want:
1) Skies as dark (light pollution free) as possible, ideally 200+km from large cities, 50+km from major rural centres.
2) Clear skies with good transparency. Often after clear sunny days moisture condenses out of the atmosphere at night, covering the sky in a light (invisible) haze that reduces transparency.
Even with all of the above, galaxies are still faint objects. So you'll want your eyes dark adapted, and take your time observing.
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