You will need to plug in one at a time and assign each one a unique address using the holding register shown in the documentation. As soon as you write the new address the device might not be accessible through the default address. Once all the devices have been reprogrammed they can be added to the serial bus together and read individually using the addresses assigned.
In regards to switching protocols, modbus is a very robust protocol but limited in the amount of data that can be realistically transferred. As long as you keep reads to reasonable intervals you shouldn’t have an issue. I don’t know what the limits are precisely but I misplaced a decimal trying to read a batch of 24 or so inputs, something like 10ms vs 100ms or maybe 1000 vs 100, can’t remember exactly. That lead to some very buggy behavior on that device.