I think my query is unique. I actually want to combine General with Custom formatting, but it looks like it may not be possible.
1. The comma is a visual aid to show the order of magnitude. If I look at a number such as 4125859 it is not apparent to me whether it is approximately 4 million or 4 hundred thousand or 40 million. Whereas 4,125,859 immediately tells me that it is approximately 4 million.
2. The General format is there for smaller numbers with may decimal places.
I've tried Custom formats such as "###,###,###.#########" or variation to this. It works fine except for whole numbers which show up with a decimal place at the end, which I don't want. For example "1245" shows as "1,245." whereas I want "1,245".