I am in the process of buying a new wheelset for a fixed gear bike. There are so many to choose from that it's hard to decide. One thing I've noticed is some wheels have hollow axles, while others are solid. Would it be okay to use hollow axles without quick-release skewers (axle nuts instead)? Also, is it possible to change the axle in a sealed bearing hub? If so, how difficult is that?

Thanks.