Edit: Also, as far as the BB types, since omniums use external bearings, they're extremely stiff because they bolt directly to the other arm. Square tapers are weak and whack (IMO)... people bone Sugino 75's because they say NJS on them. If you put a 225 lb guy on square taper cranks and have him stand on them horizontally, you can see the arms flex down.
What? This statement is absurd; square tapers have proven themselves time and time again in almost any cycling scenario from road/track to MTB. Tell me how a hardened steel square taper spindle is the cause of flex vs a splined crankset. Any flex your seeing is out of the crankarm and/or loose ball bearing deflection, not the spindle. NJS has nothing to do with this conversation.
As far as bearing break in, taken directly from phils wood's site:
"hubs have a break-in period, which can last up to 350 miles"