i guess the standard genre tag is ok, what's different with an album genre tag? think i don't have many multi-genre albums or i can just put my own in the genre tag already by modifying it with right click.
If you're sure you're only going to have one genre tag per album, you have a relatively small set of genres (like <25), and you're not going to change your mind later and do something with multiple genres and/or genre categories then by all means stick with the main genre tag. I was thinking of it similar to album artist, where you need a single value for the whole unit rather than identifying each track specifically.
how is album artist tag retrieved or managed in picard and mbee?
I'm not sure what you mean. Either Picard or MusicBee's auto tag by album feature will set a single album artist for the whole album (unless you've messed with the configuration so that they don't).
the point eventually is how do i retag all this library if i have many possible variations like artist/singles & ep/blabla single/track x or artist/x collection/cd1/symphonies, etc etc. i can't move everything!
Exactly why I recommend staying away from genre for file organization, it gets too complicated very quickly. What do you do when one artist performs in multiple genres, let alone when you have compilations and such? If I wanted to find my files by k.d. lang, I wouldn't know whether to look under Country or Pop or Jazz, and that still wouldn't find the compilations. Whereas within MusicBee I can easily find them all in seconds without caring how they're organized.
If you set up a basic template you're happy with, then add exceptions for singles, eps, multiple discs, or whatever you want done a little differently, once you're happy with the results you can walk away and let MusicBee handle the details. (Although I am assuming you're not often changing the album or artist tags on a track.)
Personally I just use Disc-Track# for multiple discs, which gives you 1-xx and 2-xx, etc, in the same folder - no need for exceptions. I know some people like to have separate folders for albums, eps, etc which I don't worry about but seems logical enough if you know what you're looking for. For me it's all about how quickly I can find the files I'm looking for, if I honestly have to find them the hard way rather than with MusicBee (which rarely happens).