Different scales in one song?!

Hi...it's me again. I sent a thread a couple of days ago about the theoretical basis of 'Private Investigation' by Dire Straits - are you there, chris mood? - , and things became more clear to me, just to get more dim. I am wondering: are there any rules to change between scales? and how can I use different forms of the same chord (e.g., D major and D minor) without sounding dissonant? To be more practical, this is 'Your latest trick' by Dire Straits....how could Mark Knopfler use the C# along with C#m (which is a part of the G# phygian scale)? Forgive me for my ignorant-looking questions, but I would really appreciate your help. Thanks everybody....


E
All the late night bargains have been struck
F#m B A/C# B C#
Between the satin beaus and their belles
F#m
And prehistoric garbage trucks
D13(#11)
Have the city to themselves
E
Echoes roars dinosaurs
F#m B A/C# B/D#
They're all doing the monster mash
C#/E# F#m
And most of the taxis and the whores
D13(#11)
Are only taking calls for cash

A B
I don't know how it happened
F/G# Aaug
It all took place so quick
A E/B E/G#
But all I can do
F#m/A B A/C#
is hand it to you
B G#7
And your latest trick

O.K. I took a quick look at the chord progression and here's what's going on. Your in the key of E major, which would account for the E, A, B, F#- chords.
The C# major chord is the five chord of F# minor, so technically you have a quick modulation into the key of F# Harmonic minor.
The D13#11 chord is not that dissonant (thanx to the #11) and resolves nicely to the Emaj.
The G#7 chord at the end would be the five chord of C#minor, which is the relative minor to Emajor.

There are no rules to theory and song composition, only guidelines to help us create, understand, and converse among each other.
Good Luck!

The rule is: There is no Rule! Trust your hearing!