ESP8266 as I2C client
I assume that you are using the Arduino core for the ESP8266 (https://github.com/esp8266/Arduino)


  Am I right that ic2 will not work?


I am not certain, but I get the same impression as you.  It seems that the Expressif SDK only supports master mode.


  If I am right is there an approved way to do ESP to ESP serial comms.


There is no "approved way", but there are many ways you can do it.  A direct serial link will give you a simple pipe of data - what you put in at one end comes out the other.  It's up to you to decide how you format and interpret that data.

My ICSC library may help you with that - I use it to connect an ESP8266 to an BeagleBone Black through serial: https://github.com/MajenkoLibraries/ICSC