Here is a picture of a kolony head on a cadian:
There are a few more in my gallery if you need. They are noticeably smaller than the stock cadian heads - I don't consider this a problem as the cadian heads are pretty damn big.
IMO they look a bit more in-scale than most
GW heads and the difference should not be noticeable at arms length.
I don't think cutting the gun out of kolony militia would work at all. I have never bought any myself but they are all solid metal minis. You're better off either shaping/adding bits to their guns to make them look
40k, or using
GW models as a base and adding heads. I have seen a bloke take some of the kolony militia with chunkier guns and drill holes in the end to form meltaguns (for example) and I'm sure that many of the pig iron models could have
GW weapon bits added on. But don't try to cut the whole guns away from the models.
Hope that helps.