1) That’s right. A tracker ID have to be unique within one PSN server so the receiver can distinguish them properly. The specification doesn’t impose to have different IDs across many PSN servers. The receiver can distinguish different trackers that have the same IDs by the source server they come from, and remap the IDs if necessary.
2) Any ID that fits 16 bits is valid, so 0 is a valid ID. Note that the ID is not necessarily an index, which means that you can have a PSN server sending 3 tracker with IDs 23, 1004, and 547.
Hope it clarifies.