I'm also having the problem with the space after a comma in decimal numbers (like discussed here: German language: use of comma in numbers)
and I wonder if I can use both the icomma-package (which seems to be quite old) AND siunitx together.
I'm not sure if that makes sense, but I have a large writing project (Ph. D. thesis) which already contains a lot of numbers.
With siunitx I'd have to use the \num[]{} command for each one of them and I would not dare to replace something like that automatically, so I'd have to check each single number - that's impossible at the moment.
So I'd like to at least have an automated solution for the numbers used in math mode and that should be icomma - right?
My only concern is, if both packages could create a conflict - is that possible?