A 460cm mast will cover you adequately in the 6.0 to 8.0 sail range, and your mast extension will allow you to adjust the mast to obtain the proper height for each sail. But, to ensure that your mast extension is long enough, just make sure it when matched to the mast that it will extend to meet the luff length of your largest sail. Chinook mast extensions, like their booms, come in varying lengths, so you can easily pick the right one. Just be sure that you have approximately 6-8" that will insert inside the mast to provide the needed interface and bearing surface to prevent mast breakage in the bottom section.
Now, will the 460cm mast work in sails smaller or larger than the 6.0-8.0 range? Yes, its possible, particularly in smaller sail sizes, but the stiffness and the mast bend or curve start working against a sail's design and character it things get too out of wack. If you go to the Sailworks website, they've got a mast matrix for each of their sail designs that shows what's workable. If you stick with cited recommendations, you're pretty much guaranteed that you'll have a suitable outcome.
Regarding the best sail sizing, I would think that either a 6.0/8.0 or a 6.5/8.0 combination would pretty good. A 6.0/7.5 combination would also be quite good. However, as sails get larger, they can be a bit more to handle when uphauling, waterstarting, and jibing, especially for a learning windsurfer. To help discern whether a 7.5 or a 8.0 would be best, I would recommend visiting the beach you intend to sail at and talk with some of the locals and see what they think. Just remember, in the beginning you're going to be primarily focused on the smaller sail to develop the needed skills, and that you will eventually expand things to include the larger sail, especially as your desire to be planing in the straps grows.
In the long run, if you become a dedicated windsurfer, it's very likely that you will expand your board and sail quivers, to include added masts and booms of differing lengths, but until that time you'll be set for a sound start with the Go, a couple of sails, the 460cm mast and a single boom.