Tinbum

Yes

If you want to use the USxxxxC you can but you can only use 7 of the old type. You can have up to 16 batteries in total then without a hub. To get 9 working he's part way there.

The manual tell you how to connect them up and in what order and the max number of each type. You can have 16 batteries without a hub but only 7 of the older types. So you will have to dispense with 2 of your older ones to use your new ones.

I would never estimate SOC on Voltage, it's very inaccurate due to the charge curve of batteries. mV difference in voltage can be a massive difference in SOC.

Yes and it's true.

Some BMS's like the JK require the dip switch 1 to be set on (id 1) to enable communication with the software. Nothing to do with how many batteries.

I would change your topic heading. Also give details of the app your using.

A simple search on this forum will bring up the protocols!!

I use a Teensy to read the data from my Pylontech batteries on the CAN line. You will need to make sure that the speed of the pi is fast enough to not miss packages. I'm sure you will find the protocol for your batteries with a bit of googling online. I would set the pi to just snif the data first and then you will get an idea. I find CAN much easier to use but it often doesn't contain as much detail as the RS485 or RS323 (Growatt batteries are an exception to this). Regarding putting the batteries in series. First are they designed for it? If not you will have to devise a way to balance each individual PACK or as soon as one pack reaches 100% SOC the others will stop charging. Not necessarily due to the bms but due to the battery chemistry.

Is it's SOC vastly different?

You want to download the log files with BatteryView and check them.

The dip switch has to be id 1

That doesn't look like a jk bms.

Same one.

If you look at the RJ45 you should be able to see which pins are populated.