#1
I want to know how to form my own chord progressions. So, do I just choose a scale then use chords with notes from the scale? Also if it was a minor key could it have major chords in it and vice versa?

Sorry if I'm a bit vague, also what is a Dominant 7th chord? Is it the same as what's referred to as " 7th chord" ? I know how to play some 7th chords in open and barre form but I'm not sure of the proper term
#3
I know basic music theory. I just wanted to know how do chords that are minor fit into major keys and vice versa. Is it this right here?

This gives notes D F# A. This presents a problem - F# is not in the key of C! In order to keep this chord in key, we have to flat the F# (lower it by 1/2 step) down to F natural. This gives D F A, which is scale degrees 1 b3 5 of the D major scale. 1 b3 5 is the formula for a minor triad. Therefore, your second chord is D minor.

http://www.ultimate-guitar.com/lessons/chords/what_chords_are_in_what_key_and_why.html

I actually understand alot better now

the text is from the link
#4
Well for a major key it follows a progression because the key signature forces it to.

For Major keys it is I ii iii IV V vi vii' I where capital I is a major chord and i is minor chords.

So C major.

C major d minor e minor F Major G Major a minor b diminished minor C major.
Those are the chords found in the Key of C major.

It works for any key as well...Major of course.

Minor keys are a bit harder because sometimes it is required you raise the third in a five chord... It mostly depends on melody.