Yes, exactly, DrivePool always uses the disk with the most free space without real-time limits.
And it does do that from what I've just tested.
See:
Here, I've started copying 100 to 200 MB files onto the pool, enough to start filling all the drives.
At first it started balancing over to E:\, then to I:\, then equalizing to all the pool parts.
Balancing settings are: