What you want to do is use the custom particles feature. To do this with random letters, you would need to make a 26 frame composition 150 x 150(or so) and place a letter. Apply a text animator-offset and animate from 0 to 25.
Drop that into your final comp and under particle change the particle from sphere to custom particle. The problem you will encounter is that letters are odd shapes, have holes in them... You can hide some of this by making the initial size 2 so they overlap and animating the size as they are dispersed. If I were doing this, I would probably have a quick dissolve between my final logo and particles, maybe hidden by a lens flare.
I quickly modified the red giant project to show you how custom particles would be incorpated.
"I have not failed 700 times. I have succeeded in proving that those 700 ways will not work. When I have eliminated the ways that will not work, I will find the way that will work."
---THOMAS EDISON on inventing the light bulb.