Hey All,
I'm really busy at the moment, so this answer won't be as usual as they usually are...
1) When you are in a key of a song, any mode (be it dorian, lydian, phrygian etc) will contain all the correct notes for the key that you are in, it's just the starting point and the sequence of notes that give it character. Meaning while playing the progression I IV III, you could in essence create a melody just using C Ionian scale (C Maj scale) because all the notes WILL work, but the melody will be textureless... There's a reason for alternative modes. Let's take a look at the Phrygian mode. The Phrygian mode (of any maj key) contains this formula 1, b2, b3, 4, 5, b6, b7. So Phrygian is the 3rd mode in chord harmonisation in the key of C, so it will be E Phrygian. Now the quality of the scale is derived from it having the b2, b3, b6, b7. If I've lost you on this one, let me try and break it down...
E is the 3rd note in the key of C, meaning it's scale is Phrygian. (now presuming you know your key signatures) we all know that E contains the notes E F# G# A B C# D#, now you could imagine how aweful using those notes to play over the key of C would sound with all that dissonance, so to create the Phrygian Mode, we flatten the 2, 3, 6, 7 to make the mode fit in with the KEY OF C, so that there are no out of place notes.
I know this is long.....
The tonal quality of the Phrygian scale is a flamenco style, so you can see that using different modes to create a melody or solo is important, not necessary but important.
2. You're question doesn't make sense... If you were in the Key of C and you created a melody using Dorian, that means you would be using the D Dorian scale (second degree in the key of C) to create the melody and that contains the notes D E F G A B C, if you created a melody using C Dorian, that means you'd be in the key of Bb.... A method to use to figure out a suitable chord progression to a melody would be to analyse all the notes you use, write them down, now find out what key your melody is in. There are no RIGHT ways to determining a chord progression to a melody, but it is a good way to find out what key you're in The chord progression entirely depends on the creators ear.
hope this helps.
Brad