if i understand you correctly, that functionality is already there. To address the different rainbowduinos, you need to send a different address in your i2c commands.
like in
ShowChar(4,words[i],r[i],g[i],b[i],j);the 4 is the address of the rainbowduino.
in the manual you will see that most commands have the address as the first thing to send.
hope that helps.
if not, then i dont think i clearly understand what it is you are looking for.