No offense, but your questions are easily answered by a search.
You can definitely use the Time Capsule as a NAS and not just as a Time Machine disk. However, as TM uses the disk until it is full and it is quite difficult to partition/allocate space on the TC for TM, it's generally better to connect a USB HDD to the TC to be used as a NAS and just use the internal for TM.
As for the second part of your question, I'm not sure of the specifics but you should be able to set it up. Do a google search to find out how people connect their Time Capsule to Linux. I'd provide a link, but I'm not sure which version of Linux you are using (I found links to get Ubuntu connected to TC, so I know it's possible).