You can mix drive sizes but every drive in a raid pool will be treated as if they are the same size as the smallest drive (mixed size z1 pool will have to be done via the true nas interface)
You cannot add a smaller drive to a raidZ pool later, same size or larger only.
You also cannot turn a z1 pool into a z2 pool.
Only "freedom" we have us slowly replacing every drive with a bigger one, once every drive is replaced the system should automatically increase the size of the pool.
We pretty much have the same capabilities as truenas scale