Receurdos, I noticed people use free strokes and use the thumb for the independant notes, is it possible to use picados only, and use the index and middle for the independant notes rather than use thumb for them?
It is, and it happens, what you've seen is done for a few very simple reasons. Mainly: sound, loudness, and tradition. If you know the setting we often have to 'put up' with, you'd realize that you're up against rhythm sections, palmas, singers, dancers, and more. If you want to be heard, you need to be able to reach the same volume. Of course this isn't necessarily undoable with middle -and indexfinger, but your nails won't last half an evening. It is easier with the thumb. Also good to note, the thumb is often used as a pick, in similar manner that the 'pick' is used on the Ud. The techniques are very much alike, and the tone is very different from the other fingers.