That is pretty advanced, and if your a beginner learning modes it is hard to grasp. I know it works and it can save time when finding the right mode to play. Learning is a different question. You can't just look at a Em chord and know you what mode to learn. It's could be E dorian, E phrygian, or E aeolion. Just an Em doesn't give you enough info, visual or aural. You would need to know what degree it is in the song's key and/or an extension. So if it's your first time tackling modes, it doesn't make much sense. To me and you yeah, cause we understand what is needed to make it work. Plus this isn't a visual forum so you can see it. That's why I say make a site about it, Griphon.

The thing is it's hard for a beginner. The way to get a complete understanding of everything in music is to what I would call cataloging a musical mind. This mean everything has it's own place which doesn't refer to other things. You learn a scale, you learn the scale by itself. Without having to compare and contrast it to something else. It's like library in your mind. When you look for a book, you shouldn't have to find three other books before you find the one your looking for. So you have all the scales in their own files, and as you learn more and more about each scale. You put more stuff in those files. You don't want a scarttered mind when it comes to music, you want everything to be organized in your mind.

About the 21 x 12 thing. Once you learn a mode and you know it's purpose and how it is made. You only have to learn that mode once. So your only learning 21 modes. Another thing, when you want to play G mixolydian, without thinking you should know the key would be C. There's nothing hard about figuring that out.

About all the other scales. You should COMPLETELY learn the Major/minor modes first, then move on the the harmonic minor and melodic minor ones later. Their modes are mostly altered scales, or scales that don't find much use in modern music anyways. Simply cause, very few songs are written in those scales. Another thing, you must know a great deal about theory before you can even use them to there purpose. As for Pentatonics, they have the same modes as the Major/minor scale with some notes taken out. If you know the major/minor modes already than the transition to learning the pentatonic ones is simple.

Once you learn how modes are formed, it's easy to figure them out on any scale. Even the exotic ones.