cranks- some mfrs make 172.5s
8 spd shifters/ deraileurs- throughout line- not if Shimano yes if campy and some others
different shifters/derailleurs with different cranks- no problems
advantages different count on chainring teeth- depends ie: 53X45 or 42 good for flats and time trials, 53X39 better for hilly stuff.
Bike from scratch- triple chinrings, long arm derailleur; double chainrings, short arm derailleur