Some of these issues of this big topic have been hinted at elsewhere, but I can't recall seeing explicit recommendations...

a) Should separate History be maintained for each Broker used? (Obviously at the expense of disk, potentially lots of disk)
b) Does this matter less for EOD History? I.e., can any Broker's History be used for EOD?
c) Does it matter more and more the finer the Trading time period is?
d) If I read the manual right, some of the Z Systems are Tested using a simulated Oanda account but with FXCM data? When is this mix-and-match OK? At what BarPeriod or other measure does it become critical to use the target Broker's History?
e) If one has multiple History files for an Asset, possibly with partially overlapping data, can those safely be combined? Is there a helper script to do so?

And so on... As I said, a big topic...