Ving I doubt you would ever get to usefully barlow that 7mm.
One thing to remember is that the fov on these eyepieces is smaller than a plossl and very high powers will make it harder to track.
If you get good seeing conditions often you could go shorter than a 7mm . I got the 7 as Melbourne seeing is lousy.
Doug, 188x should be a good planetary mag in your refractor, and usable in the dobbie if you like getting up before dawn to observe
