Re: alpha doubts

Your best bet would probably be a Luma Key. There is a Luma Key already included in AE as an effect, and you can try it first, but you'll have more control if you do the following.
Duplicate the ink-drop layer. In the comp window, look at the individual channels. Make a note of the one with the highest contrast. Then apply the Shift Channels effect. Leave Alpha alone, but shift the other channels to the one with the highest contrast. You want the drops on the layer to be pure black and the background to be pure white. If not, add a Levels effect and push the left and right triangles in towards each other until you've achieved the highest contrast possible.
Then back in the timeline, on your bottom layer turn on the Luma Inverted track matte (TrkMat - L.Inv).
And there you have it.
Sometimes you'll need to tweak the levels command, particularly if you have a lot of edge detail you're trying to preserve. And sometimes I find that in the shift Channels effect, it works better to shift all the color channels to Luminance. And of course some footage will require garbage mattes as well if you have extraneous elements.

Joey Foreman
Editor/Animator
Nowhere Productions, Athens, GA