first off, you can't map tags into JRiver at all if you post all caps in the subject line of your posts
Nan seriously for those fields there is no issue, it should be seamless (both use Vobis comment tags for FLAC) provided you have not created custom fields, but they should be remappable within reason (choir for example might need a special field; better to use the standard soloist field instead of performer etc.). Soloists, Conductor, Orchestra etc are standard tags that may look different in the tagging window but are not ... example : TRACK will lok like Name and ALBUMARTIST will look like [Album Artist] in the tag window of JRiver ... maybe this link would be helpful to you?
http://wiki.hydrogenaud.io/index.php?title=Tag_Mapping#Mapping_TablesFields like OPUS dbpoweramp doesn't download any way, so if you want to tag that (I create a custom field [Work]; its better to set up a new field in my opinion; tag it in JRiver setting the option to write to the tag in the file itself. If you use custom tags in dbpoweramp, just make sure they exist in JRiver first (or create them in options). No need to paste anything.
Generally what I would recommend (at least at first) is to choose the metadata you want that is provided for via dbpoweramp; then retag in JRiver rather than adding in information in dbpoweramp before.
So to answer your question I would set up profiles in dbpoweramp one for classical and one for everything else. I can paste mine if you want. You could copy/paste it and modify it from there. Set-up your autoimport config to include the paths that correspond to your ripping "masks" -- I "sandbox" and dump it into a temporary path, but depends on your workflow of course. Then set-up a tagging view to retag if necessary that includes the fields you want to maybe modify or at least double check.
Don't forget that depending on how you use the tagging window in MC, you might not see the tag fields. Click on the little icon at the top left of the tag window to show more.
PS - Oh one thing I wish I new a long time ago ... in the metadata configuration for dbpoweramp, click them all ... you might need to use composersort, artistsort etc later ... its possible to semi-automatically populate these fields, but its a lot simpler to have the xtra metadata even if you do not use it. The space taken up is so small and if you become a "geek" about tagging later you might appreciate having it there.