Greetings,
Question 1. If you navigate to micriums website (the company behine uC/OS_II), you can find all the processors that they support. Of course these are just the ones that they already have ports for. If you were to write your own bsp then you could use it on other processors as well.
Question 2. Unfortunately, uC/OS-II is not free for commercial development. However, if you are just using it for personal projects, or if you are a student, then it is free. However, this is just for the source code. Other features, such as USB stacks, filesystems, etc, are not free; even for personal use (though I do think they provide free trials).
Question 3. Yes you can work with Windows 7. The more important question that you should ask is what platform and IDE you want to use. But yes, Windows 7 should work fine.
Question 4. What arm where you planning on using. As I mentioned, you can go to Micruims website and they will list all the processors that are currently supported by uC/OS-II. When you decide what processor you want, download the source code and the port and they will have an example project that will usually allow you to blink some LEDs or something. This will get you up and running. From there I would recommend looking at documentation or maybe even consulting one of the many books that are available regarding this rtos. Good Luck
-Willis