I would not expect any compatibility problems (especially with the two programs you list) with any of the following fields:
But you have to understand what kind of data is expected in a couple of these fields.
YEAR: Stick to four-digit years (dates) and you'll be fine. Don't add the month or the month and day (e.g. "2002-04" or "2010-11-28"), just use "2002", "2010", etc.
TRACK: Don't include the track total within the TRACK field. Sometimes you'll see "1/12" or "02/14" or even "1 of 10". Also, don't pad the number with zeroes. It should be a simple integer.
GENRE: Mp3tag will allow you to enter multiple genres for a track, but not all programs will see all of them, so it's probably best to use only one GENRE for maximum compatibility.
A couple others that are very useful and will not cause any problems in those two programs are:
DISCNUMBER: Same caveats as with TRACK.
In my experience, it's very rare that extra, unrecognized fields cause any problems. Most programs will simply ignore fields that they don't recognize. Maybe not all, but the vast majority.
The beauty of Mp3tag is that if you discover a problem somewhere in the future, you can easily do mass tagging changes of your entire library. So don't worry about it too much right now. I'd highly recommend adding as MUCH metadata as possible and if you actually encounter a problem, fix it later.