How to make a custom filed forced to be found in one of the tag group (e.g. Classical)?
Customise the Tag Window to include any fields you want, in any position you want. See his post for hints:
https://yabb.jriver.com/interact/index.php/topic,118271.msg818141.html#msg818141So modify the Audio template, or probably better yet, add a new "Classical Audio" template, restrict it to be used only for classical audio tracks, add it before the existing Audio template, and configure it the way you want.
Of course, using a new "Classical Audio" template implies that you have a way to differentiate Classical Audio from other Audio tracks. There are lots of forum threads and some Wiki posts regarding that for ideas, if you don't have something set up already.