On U1 series, the maximum of Rsync tasks/targets are 8/4. On U2/U3/U4/U6 series, the maximum of Rsync tasks/targets are 16/8.
No, each SSD drive can only be assigned as SSD cache disk for one pool.
The SSD write cache on Qsan NAS is only useful for improving Sync write access. If it's all Async writes performed to the pool, then the overall performance would not benefit much from the SSD write cache usage. For more details please refer to the SSD cache deployment application note from Qsan partner portal.
On Qsan NAS the compression algorithm works based on LZJB algorithm natively provided in ZFS.
Zero reclaim feature allows  the system to automaticaly  reclaims continuous blocks with zeros (of 1GB granularity) and mark them as usable space to be leveraged by other zfs. Once reclaimed, the reclaimed space will be calculated and added to the available space of the pool, thus increasing the overall usable pool capacity. The mechanism of this feature works similar to only compressing zeros and is therefore listed as a compression option.
Yes, by enabling the compression options, the system performance is always impacted as the compression/decompression process would somehow occupy system resources at certain level.
From the Web UI, the configurable ACL permissions on the share configuration page refers to POSIX layer ACL. And the Windows ACL works as an additional set of ACL attached to the folders and files.