From what I have read.....it is OK to mix the G-05 and "traditional" green.
HOWEVER, the G-05 will not be "long life" of 5 years or 150K miles......
The G-05 will have the life of the green coolant....which is 2 years.
This is also something to keep in mind if switching over from the traditional green to a long life fluid...such as G-05.
If all the traditional green is not flushed out, then the long life charactoristics of the new coolant are lost.
I do not consider this to be any issue that would prevent me from using the long life product.....but then, I don't plan to leave a coolant in for the long life period anyhow.
Also, G-05 is a great product.