If I create a song using entirely original loops and melodies on MMM with the use of only a few premade loops for sound effects, crashes, reverse crashes (nothing that's looped for long periods of time), do I own the rights to the song?
This topic has been covered many times before. Use the search box on the right for Copyright and also review the Legal Information option on the right.
However, if you are using any soundpool loops, you are bound by the limits of the license you hold. You need the read and understand the license. As the licenses say, at no point do you "own the rights" to any of the soundpool media. You merely have a license (permission) to use them only within the limits of the license.
You do bring up a very interesting point. However, as far as the soundpools are concerned, MAGIX does not differentiate single percussive hits from any other loop.
You "own the rights" to only that which YOU have created.