I Fingerpick with my thumb curled down, is it supposed to be straight when I pick with it or what ?
If you can play songs using your current technique, there's no problem. My thumb actually points up when I fingerpick... it's really just finding what's comfortable for you.
Well it depends. For certain songs (and therefore for a certain sound/style) it won't matter, but other times it will. Personally, I always keep my thumb straight, but I also rarely use my index finger, so....yeah. It's mostly a personal preference thing.