Putting a Calibre library on a NAS and using it from multiple OS is no issue as long as one is aware of the issues around the fact that Calibre is single-user, and long path names can be an issue.
I have my Calibre library on a NAS and am accessing it from Windows 7, Linux and OSX at different times during the day without any problems. The base path length on the NAS is short so that all OS should work fine with it. Directory separator characters are handled automatically in Calibre in an OS independent way.
I find that the one issue that used to really bite me of the filing system being Case Sensitive or not rarely seems to rear its head any more. I think that is a combination of Calibre having got better at makign sure case on the Filing System matches the database combined with me having picked up workflow habits that mean I avoid the typical problem cases. Having said that I do periodically run the Calibre Library Maintenance option to check I am not building up any hidden issues.
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