My ceiling is about 6'9" so a little shorter than yours and I bought the 500mm extension to replace my B pole. With shipping and taxes the extension wound up being around $74 USD for a brass pole. I combined the 500mm extension with my A pole, and the 250mm extension that comes with the pole. I guess I could have gotten away with the 750mm extension...lol! I can see the red indicator line below the adjuster which means the adjuster at the bottom of the pole is at 100mm. X-Pole will be able to advise you which extension is right for you - their customer service is amazing, and I worked with them to find a solution that was right for my pole room! :)
If you want a teensy bit of extra space, replacing the dome with a permanent mount might help, however, this involves a bit more installation as you have to screw the mount into a joist. It might also be helpful if there's a temporary pole space in your house with a higher ceiling - a room where you could pole sometimes if you wanted the extra height, but maybe can't keep your pole up there full time. Using a permanent mount will take the hassle out of installing your pole so you're free to put it up and take it down easily. I bought a permanent mount but my fiance won't let me install it, so maybe others can chime in with more insight.
My ceiling is a few inches shorter than yours and I'm still able to spin, dance and invert (I'm 5'1", by the way). Although it's workable, I do find my low ceiling a hindrance when it comes to some moves, and basic climbs are not something I can do at home. Another thing to consider is if you're getting a crash mat or using heels, you're going to lose a couple of inches, too. If I had the option I would have my pole set up in a room with higher ceilings, but you do kind of get used to it, and it does force you to be a little more aware of your surroundings, and a little closer to the ground. ;)
Lots of ladies on here have poles set up in rooms with low ceilings, and they're all able to make it work. I'm sure they have much better tips than I do!