Alright my bad people... according to the diagrams we have in the doctor section, the lowers of the G17 and the G18C are quite similar if not identical, even alot of the part numbers are the same...
Perhaps I just had a pair of gun parts that were being difficult at the time, but nothing in the diagrams indicates that it would not work.
Now unfortunately I could not find views of a 26C, only the 26. But seeing as the 17 and the 18c appear to be good, the 26 and th 26c would be good.
I realise that I have only offered alot of speculation, but hopefully this helps in some way.
Possibly someone on the boards has actually done a swap and could enlighten us?