I think the answer is:
Note
My arrangement of the riddle is:
Many of me define the key but you must be picky
Talking about a musical key, e.g. for a guitar each key is made up of 7 notes "picked" from the 12 notes available
I'm a type of a galaxy
Referring to the Samsung Galaxy Note range of smart phones
I can be sticky when I'm in stacks
Good old stacks of sticky post-it notes
And you must pay a tax when I'm ripe
I'm a little less comfortable with this one. I think it refers to a debit note which becomes "ripe" when it is due and a payment is owed.
Disclaimer
My musical theory is pretty shaky (non-existent), the reasoning behind the key might be a bit dodgy. It is based on some rather rushed research and this article.