ym8016

This has been running for the past four years without any issues. You only need gas CoC after the installation of the gas water heater, not electric CoC. Nothing will blow up even if you go from 0 to 75 deg, the gas water will ignite and heat up water and when water from the electric geyser at 75 deg gets to the gas water heat, it switches off and just let the hot water from the electric geyser to pass through.

Most definitely you can do it. Will you get a CoC (electric and gas) for it, not sure. Will it blow up, probably only if you go from 0 to 75 degrees inlet water quickly.
I don't have the manual, but check if there are high inlet temperature trips. Otherwise you must devise an electric cut-out for the gas heater if the electric geyser temperature is too high.

If you're talking about the Growatt SPF 5000 ES, then I'd personally I'd look at the first string being 5 Megatron in series with 3 JA, and the second string being 5 Hyundai in series with 3 JA. But also, that's if you're satisfied to connect just 7.25kW of panels when the inverter(s) can handle more. Just a personal opinion, to be checked professionally.
The disadvantage would be in that you cannot cannot effectively approach the maximum 6000W solar array that each inverter can handle, or efficiently upgrade after that, unless you use a different panel that's more closely matched to the 18A MPPT spec. In theory the additional 4 Megatron panels will mostly not quite cover charging the additional storage capacity.
It's your call to check/say if it meets your needs; there's not enough info about your daily loads and off-grid/on-grid situation to form a proper opinion.