To clarify a couple of things here (sorry, I did skim here):
StableBit DrivePool's default file placement strategy is to place new files on the disks with the most available free space. This means the 1TB drives, first, and then once they're full enough, on the 500GB drive. So, yes, this is normal.
The Drive Space Equalizer doesn't change this, but just causes it to rebalance "after the fact" so that it's equal.
So, once the 1TB drives get to be about 470GB free/used, it should then start using the 500GB drive as well.
There are a couple of balancers that do change this behavior, but you'll see "real time placement limiters" on the disks, when this happens (red arrows, specifically). If you don't see that, then it defaults to the "normal" behavior.
