#1
Do you ever reject an idea, a riff, melody, chord progression because you think it sounds too much like something else? I used to do this a lot. Now I think that was an unnecessary brake on creativity and progression.

My advice to people starting writing music is not to worry too much if it sounds derivative. Perhaps listen to more varied stuff to muddy the influences a bit. Your own voice will come through in the end, by a process of evolution.

Good advice?
#2
I can get behind that.

It's a similar principle to prescribing a group of musicians with the same bare-bones material, whether it be a melody or even two chords; each and everybody will interpret the information differently according to their experiences, tastes and skill, and produce independently voiced material. It's not so much the source, but the person working with the source.
#3
I feel validated when I write something that sounds like someone else. Chances are, they're better than me and by writing something that takes from their style means that I've a) written something that probably sounds good and b) taken a piece of their style and put it into my own toolbox which is the ultimate goal, to take as much as possible from as many people as possible. Now, if everything I wrote sounded like Shostakovich or The Jonas Brothers, I'd be concerned, but if it's just aspects, I'm usually very happy.