Basic music theory questions?

1/ No. Usually a melody will use only notes in a given scale (which relates to a specific key), so that a piece of music in C will use the notes, CDEFGAB, but other notes called accidentals can be used to. Chords aren't "in" a scale or a key. In any key there are chords that are formed from the associated scale andare more commonly used but, in theory, any chords could be used.

2/ I don't think and scales are more important than any other but some are more frequently used that others. The basic major and minor scales are the most important to know.

Before you go off onto a furious crusade for knowledge, you should learn the concepts behind the information so that you can actually apply what people tell you.

Every major key will have 7 notes. We'll start with C.

C D E F G A B

From each of those 7 notes, you can start a mode of the scale *using all the same notes*:

D E F G A B C

E F G A B C D

and so on.

Every key, as far as intervals and patterns go, is set up the same way. You'll find the same number of minor chords in C major as you will in D major as you will in E major and so on.

Whatever your instrument, pick ONE major key and learn all of its modes. Then practice those patterns. Then apply them to other keys. Once you have this foundational knowledge, you'll be able to form virtually any blues, diminished, augmented, whatever scale that comes your way.

I'm about 80% on this, I believe 1) is no, as I have seen music in certain keys that have notes that are natural when the key is flat/sharp, but I think chords are usually in the same key. 2) the major and minor scales, being A maj/min, B " ", C " ", etc. The blues scales are usually only used in jazz/blues/swing music so unless you're specifically studying those, they shouldn't be too important.