I've finally got it working... It was the initialization for the Oleds causing them to display too small....
I used:
U8G2_SSD1306_128X32_UNIVISION_F_HW_I2C u8g2(U8G2_R0, /* reset=*/ U8X8_PIN_NONE, /* clock=*/ SCL, /* data=*/ SDA); // pin remapping with ESP8266 HW I2C
// Initialize I2C buses using TCA9548A I2C Multiplexer
And now all working!
It was trial and error but basically used one at a time from the example code provided until it worked correctly.
Thanks for the advice everyone!