I'm new to all of this too.
I would think that for most bows you would simply need a puff as an accent.
in which case individual puffs would be the way to go if you can get hold of them. Otherwise there is a very recent thread that tells you how to make puffs from the "linear" (for lack of a better term).
Basically you take a 1.5" - 2" section, form a circle, and glue it together. If I had a better memory I would give credit to the person that posted the information, but I'll thank them again here.
I'm not quite sure when you might need a longer section in a bow. It would have to be a pretty big bow.
And I don't think the individual feathers or boas are really the way to go unless you want a more obvious feathered look, as opposed to a downy/fluffy look.