Woody

No you shouldn't have any compatibility issues Pylontech is quite forgiving when it comes to adding modules. Should you have issues i am sure it will be minor issues nothing you won't be able to sort with some advice from forum members.

AFAIR, the Surlok connector on the pylontechs are rated for 120A. They only support crimping to 25mm², so I would double up.

The 4 x Pylontech Us3000c can discharge at 133 amps the 35mm² cable will be adequate no need to double up 35mm² cable. The chart below is based on approved SABS 1574 standards and ratings:

I don't believe this is budget yet. I saw it released not on City stationary but that of an opposition party, by a provocative councilor. It's more likely a proposal, yet to be voted on and before the mandatory process of public consultation. 
Anyway, I am expecting a flat monthly fee. It's been on the books for years (the party that put this document out wants us to forget that they tried it first, then backed down). I suspect that the idea is to squeeze prepaid users now that there enough of them who cannot easily convert back to a conventional tariff, and it will work out more expensive but not quite as expensive as the alternative. So you sigh and live with it. 
Once they put the tariff charts showing the steps then we can each do our exercise and see how better off or not we'd be on another tariff. 
Eskom have also been talking about a fixed fee for prepaid, on the usual grounds: it costs just as much to provide the connection. 
What I don't like is TWO charges. The capacity charge is for providing a connection. The service charge is for meter reading and billing. They don't have to do that with prepaid customers, so there shouldn't be a service charge. The capacity charge I grumble but I see their point. The service fee makes no sense. 
What is overlooked is that prepaid users pay in advance. Duh! Of course! This is good for the City's cash flow (not the first time I've made that point) so I hope that during the consultation phase, lots of folks ask that question: why are you punishing people who are good for your cash flow?
In the meantime folks who contemplating the switch should hold their horses until the new, approved tariff tables are made available. 

Update... I'm being sent from pillar to post between City Power and City of Joburg. It seems their systems cant talk to one another. CP is insisting that their system does not show my payment for the meter installation and telling me to go speak to COJ. COJ is insisting that my payment is showing on their side and that the problem is on CP side. I offered to provide the receipts to the CP call centre but they are not interested... so now I'm stuck between the two and of course they are not prepared to talk to one another.
I've enlisted the help of my ward councillor to help; my last resort. 

No luck .... Even set for 2.4ghz the dongle is still just flashing red and doesn't show on my phone or connect to the router?

Sorry never saw the replies (not following the topic). I  got nothing via email and eventually went in to the regional offices this week and paid (should have done this as soon as I got the 700 ref number). The guy said CP will call me to make an appointment.
After reading slipx's comment I phoned CP now. The payment has not been allocated to my account yet (clearly no urgency on the regional offices' part). If its still not allocated next week then I'll have to go in again just so I can sit next to them to make sure they allocate it. 

I remember getting rid of a car once. I wanted it off my name AQAP. I employed one of these intermediaries who told me that I had to present in person. So I said "OK... so I pay you fees why?". He said "meet me OUTSIDE at 8:30 tomorrow. DO NOT enter, DO NOT join any queue." So I did, and he marched me into some office, pointed at me and said "here he is. He was here and you've seen him" and the person in the office agreed that I had indeed been present. Then I get told thank you very much, you can go now and we'll call you when the paper work is done.

with regards to your battery vs inverter size, i was worried about the same. but it is doable, wev had our system for just over a week (not nearly long enough so treat my points as assumptions not proven fact yet). so far we have had no trips, we have also had no grid power for 4 days due to eishkom, and despite this, still no trips. we can draw 3.5kw (2*3000c pylontechs) at a time on an 8kw inverter, but we do have egoli gas so in the evening our heating needs aren't very hectic and in a our household (4 adults and 2kids) it has been easy to ask everyone to be careful of running either the iron, electric kettle  or microwave at the same time which are our only real big draws. to be safe though i got 3 tuya plugs from takealot, rated for 16a, in the evening if the iron is on the kettle and microwave plugs turn off, if the microwave draws more than 700w the other appliances cant be turned on etc. its not perfect as the tuya plugs still take a few seconds to pass commands via the cloud (local tuya via home assistant would be faster im told). saying all that i think one more battery at least, to take the draw to 5kw and allow for a margin of error is my next priority

with regards to your battery vs inverter size, i was worried about the same. but it is doable, wev had our system for just over a week (not nearly long enough so treat my points as assumptions not proven fact yet). so far we have had no trips, we have also had no grid power for 4 days due to eishkom, and despite this, still no trips. we can draw 3.5kw (2*3000c pylontechs) at a time on an 8kw inverter, but we do have egoli gas so in the evening our heating needs aren't very hectic and in a our household (4 adults and 2kids) it has been easy to ask everyone to be careful of running either the iron, electric kettle  or microwave at the same time which are our only real big draws. to be safe though i got 3 tuya plugs from takealot, rated for 16a, in the evening if the iron is on the kettle and microwave plugs turn off, if the microwave draws more than 700w the other appliances cant be turned on etc. its not perfect as the tuya plugs still take a few seconds to pass commands via the cloud (local tuya via home assistant would be faster im told). saying all that i think one more battery at least, to take the draw to 5kw and allow for a margin of error is my next priority

I suppose it's better than the R0.00 per kWh that we get in Pretoria...

If you get an inverter that can be paralleled then you will need to add a second unit that is the same as the first. I.e., you will not be able to add a 5KW to a 3KW but rather a second 3KW to match the first 3KW.