CrossRoads, thanks again! 
Yeah, I guess I was making too big of a fuss about the software multiplexing, it's not the end of the world after all
One last "dumb" question: You mentioned some pull-up resistors which should be 220 Ohm. Which one are those in the diagram and what is their role?
I think I'm going to create my cube based on this diagram and based on your explanations. But I'll start with a 5x5x5 one. It's just too complex to aim for the big rabbit from start...