FP What up?

I'm learning about transposing and modulation in music to give a sense of mood change and make progressions and melodies more interesting.

I understant the whole idea of transposing to new keys, doing this I and looking at books I've noticed a lot more scales that contain the exact same keys as other scales.

Is it a good idea, or would it would if played layered different melodies over each other?
Say I have a melody in C and I want to layer a melody over it in the key of Am, it's relative minor.
Because both scales contain the same notes, despite the different tonic, would that work?
Also Dorian and Ionian scales and all the rest, can you layer different scales over one another?
Or even say a Chord Progression in one key and then play a melody, in a similar scale?

Hope this made sense