Yeah my only concern is if it will throw off the chainline. Basically I'm just wondering if spindle length is important to the operation of the drivetrain or if it's purely just personal preference. I know the BB length is important depending on the BB shell on the frame but I'm not sure if a specific length spindle is required in order for the drivetrain to work efficiently. I'd imagine you'd want a chain that's perfectly straight going from the cassette to the crankset rather than being slightly offset which I would thing would be inefficient. Again I'm clueless about bicycle mechanics so I'm just assuming a certain spindle length may be required for it to work properly.

To sum it up:

-BB length has to match the BB shell length
-BB threading must match the threading in the shell

Things I'm unsure of:

-BB spindle length is personal preference?
-Crankset has to fit the BB but size doesn't matter?
-Any cassette will do?