The SPI pins are fixed. If you want to use SPI, you must move other functions to other pins to free up the SPI pins. (You can use software SPI on any digital pins, but that will be too slow for updating displays, especially large ones).
I would imagine that max screen refresh time would be 0.1 second, as a timer counts in 0.1 sec intervals.
volts, amps prob only need to sample and display every 0.5 seconds
4 x temps prob only needs to sample and display every 10 seconds at most.
std code has a few .GIF files that it displays, but these can be removed without issue,
leaving only text
I think I need to figure out how much ram gets used as std, before I complicate matters further.
ah, ok.
moving pins is not really feasible due to the custom shield used.
guessing this is most likely why I2C is used on std kit.
(I assumed it was probably down to cost, as many other system components have been 'cheaped out' on)
thank you (and the others), for the schooling.
every bit of extra info helps.
That is massively helpful and informative.
saved for future reference. many thanks
I had no idea what those 6 pins on the bottom edge were for (until now)
are they physically connected to the pins that they look to duplicate higher up on D i/o side.
Correct
Be aware that A4 and A5 on an Uno are the I2C pins. They might not seem used but they are when you use the I2C interface.
Personally I never use numbers above 13 on the Uno/Nano; 14 equals A0, 15 equals A1 etc. It often prevents confusion.
Hi PaulRB,
My recommended 2.4" display turned up today.
seems its 3.3V.
what is best solution to use on the UNO.
( I have read some use resistors, some use level shifters, but what is best option)
Wasn't the .91" OLED also 3.3V?
Please post links to the specs or data sheets of the small and larger OLEDs.
The output on the arduino shield is defo 5v, I measured it.
The 0.91" oled works fine on this.
The 2.4" is from Aliexpress. It states 3.3V in the description.
ah, I see.
Thanks for that
(I shall own up to not having been able to enlarge that sufficiently on my ancient phone) 
Also ,on another sellers page, it warns against using them at 5v or with Uno without reducing voltage.
However, I would still like to know (for future use), what is the best method to adopt for any 3.3V devices.
My advice would be don't mix 5V and 3.3V components if you can avoid it.