Home Depot has an inexpensive surface mount GE breaker sub-panel. It comes with several single pole breakers. Some can be 'tied' together with little post kits (pins that mount between the lever/handle on the breakers). Or just buy the ones you need outright. I would look at the brand (and the model of breaker) you already have. One less thing to be different. Though if the existing breaker brand you have is $$$, I'd go with the cheaper brand (stay with well known national brands -- easier to add/change later).
For insurance purposes, use the same brand breaker the panel is.
If you have aluminum, you will need a small tube of 'alox' to apply at the terminals. This keeps the aluminum from oxidizing. One tube will last a life time. I think Home Depot carries it too.
Don't worry about the correction factor -- that's handled by the power company. 'Power Factor' is something industrial/commercial users worry about (or the power company charges them extra to fix it). They already expect a certain amount of it for residential customers (it's just not a big issue for the consumer).
A lot of compressor motors have a large capacitor mounted on them to help with start up power (actually, current) requirements (part of the discussion on start up spikes, etc). If the motor has a large (about the size of 2 D batteries) hump on it, then you have a start capacitor. Compressor motors have to start against a significant load (especially the cheaper home or low end commercial units -- just not sophisticated systems).
I have a similar set up. And I eventually did what you are doing: swapped devices on 'the' outlet, then added an outlet, then finally added a second circuit just for the compressor. Left the extra outlet though -- as someone else posted, it's a great place to plug the plasma cutter into! I do the same thing as the wood shops -- I only run a big current draw on one machine at a time. Since I am a one man shop, that's not a big deal. Now, if I ever build that little garden shed outside and put the noisy $%^&%!! compressor in it -- that would be a good thing.
