key compatibility

G'day all,
I don't get a lot of time to devote to these pursuits,so I tend to remain in the perpetual beginner realm(holder of guitar shaped object if you like),have had no formal training and tend to pick up clues wherever i can,lurk here a bit at times(though not lately).
If,for instance my vocal key tends to be A,i guess whatever music i'm attempting to play should be transposed to the key of A?....although i know that blues harp for instance is usually keyed differently(maybe a fifth away from the songs key?)
So my question is just what keys are compatible with each other,also,if for instance i wanted to sing with my daughter would we need to be a certain # of octaves apart in the same key or could she sing in a different key to me?
How are these issues attacked in a studio?.....probably the kind of thing that's right up Lump's alley methinks.....

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