Bridesmaid dilemma- Who to choose? Help!

My fiance and I are planning to get married in just over a year, and the time has come to choose a wedding party. Who knew it was so hard!
I have asked my sister as maid of honor, my best friend of six years and am now in need of more friends to match my fiances number. (All of whom we really want to be a part).
I have two new friends that i have known for only about a year. Despite this, we are becoming really close friends, and of all my friends they have been the most supportive and excited for me.
My question is, i guess, is that too short a time to know someone before asking them to be your bridesmaid? Is is palcing to much pressure on new friendships, or might it make the friendship even closer?
Any advice would be great!