Jazzy

Thank you @Tinbum, your guidance has worked. 

Being an installer I have 1x us2000 2x us3000 1x us3000C and 2x UP5000 all in one bank working fine see pic below
From the pylontech manual verbatum:
15) Compatible with the US3000C, US2000C, US3000, and US2000.
*Mixture using master battery priority:
UP5000>US3000C>US2000C>US3000>US2000
For same type of module always use the latest production unit as master.
*Mixture using battery deployment option:
Master battery (1st) UP5000
Slave 2nd ~8th UP5000/US3000C/US2000C/US3000/US2000
Slave 9th ~16th UP5000/US3000C/US2000C
Please take note of order of batteries above if and when stacking.
They all are 15 cells i'm pretty sure
Only down site is the you need to limit your draw to the smallest battery in your bank
Mine the max I can draw is 1.9kw continuously because of US2000 (FYI lithium batteries work well between 95%-35%)
1st ALL Batteries "HAVE TO BE Balanced" or else you will get an alarm on batteries
Inverter MUST communicate with Master batteries BMS

 
 

If you are using bms communication to the inverter it does not really matter if you mix different capacity batteries,they will all discharge and charge properly.

I doubt that the installers you met, know better than the guys who make those batteries. According to Pylontech support, you can mix them, just contact Pylontech support if in doubt. This is a response which I got from Pylontech, not installers,  ''And yes, you could connect UP5000 to the 4 US2000, but you need to use the new UP5000 as the master battery''. 

Yes you can, and you will have to use the UP5000s as master 

I think that @kobus joubert meant just connect the thick battery cables, no thin data cables needed or indeed possible.
MPP Solar have updated their Pylontech page again. It seems that February 2019 and later manufactured Axpert Kings and Axpert VM IIIs have support for the proper, data connect, true SOC support. All other Axpert models just have "generic support" as Kobus Joubert mentioned, and detailed here:
http://www.mppsolar.com/manual/Pylontech Lithium/Pylontech inverter setup SOP.pdf
Really, the "generic support" just says to put the cut-off voltage setting at 47.5 V (so that becomes 0% SOC), and the bulk and absorb voltage settings to 53.2 V. Zero is now correct, what more do you want? 😮  I don't think it will even report about 100% SOC when the battery is full, in fact I predict 57%. So really, unless you have ICC on a Pi, I think it is worth it to get one of the two models that support true SOC.