Used to have to remove picture hooks and repair the ‘damage’. Nowadays, I believe it’s ok to leave any picture hooks in place. I wouldn’t even bother consulting the REA.

Re alignments - Getting two same sized pieces exactly level is a royal PITA. I would go for a stepped arrangement every time. Smaller pieces look great diagonally imo. A lot depends on how colourful or whatever the pieces are.
The kicker is the string/wire/little eyelets on the back of the piece for hanging. These are NEVER exactly the same from piece to piece - hence the need for a tape measure to calculate where to put the attachment point for each piece.
Random can be fun if all the pieces are differently sized or have different frames. Or put a giant plus sign on the wall with painters tape, and put each piece in one of the quadrants close to the crossing point. That can look fantastic if all the pieces are different.
It really is a matter of you do you. No-one can tell you you’re wrong … I think there’s a song about that.