First and foremost, I'm a mix of mixes.
My first mix consists of what I have one, and what I have seven
What is subject to change,
The result of eels and a madman.
After that, I am a mix of love and music
Of grace and agility,
Of shaking and screaming.
My last is a mix of everything
Multitudes of genres
And yes, the answer is one.
It's abbreviated to 3 letters.
Hint I:
what I have one, and what I have seven: look at that blue box.
Hint II
The result of eels and a madman.: think
MarvelSpiderman
Are you
Electronic Dance Music?
My first mix consists of what I have one, and what I have seven
What is subject to change,
The result of eels and a madman.
As @tmpearce pointed out, OP has 1 gold badge and 7 silver badges. The combination of gold and silver is called "electrum". Electro is a Spiderman villain who, in one version, gets his electricity-themed powers from electric eels.
After that, I am a mix of love and music
Of grace and agility,
Of shaking and screaming.
These could all be descriptions of 'dance'.
My last is a mix of everything
Multitudes of genres
Music indeed has multitudes of genres.
It's abbreviated to 3 letters.
EDM is commonly used.
Is it a
Book?
First and foremost, I'm a mix of mixes.
A book contains a massive mix of letters and words.
My first mix consists of what I have one, and what I have seven
Chapters
What is subject to change,
The plot chan change. Who doesn't love a plot twist!
The result of eels and a madman.
Comedy
After that, I am a mix of love and music
Romance
Of grace and agility,
Fantasy
Of shaking and screaming.
Horror
My last is a mix of everything
The last page is often taken up of other books
Multitudes of genres
There are many book genres
And yes, the answer is one.
A book is a combination of all of these, brought together by the author
bar tbyq naq frira fvyire onqtrf (rot13)$\endgroup$ – tmpearce Dec 5 '16 at 17:17