"Can I add either type to my hosts and what is the capacity impact?"
You can and it will increase vsanDatastore capacity by that amount.
"If I add the 960GB disks, will vsan still work and balance all the data over all the SSD's?"
Yes, you can start rebalance task from vSAN Health UI or RVC - only data on disks over 80% used gets moved automatically otherwise.
"If I add the 1.9TB variant, will the capacity grow by 1.2TB per disk (because I already have 1.2TB SSD's) or will it use the complete 1.9TB?"
It will use the complete amount. Note it is advised to use the same size capacity-tier devices if using deduplication.
Very clear, thanks for your time, Bob.