@thomas @espectalll don't know this exact case, but Linux kernel itself us copyleft itself, so don't see why it should not be possible (also you can usually merge not-copyleft code into copyleft one, you just have to redistribute the not copyleft one under the original licence, i.e. not claim it is copyleft).

As for the general question: ask yourself what you want. It may also depend on the project.
Do you want it to always stay free/open software, even when companies include it somewhere, or do you want to allow them to use it, even if they use it in closed source projects?
I'd not say one thing is generally better than the other. It's a big matter of personal taste.