I have no doubt that this question has been answered countless times before, but my searches have not produced an answer yet.
I'm writing a library using LGPLv3 and I would like to copy a couple of utility files from a GPLv3 project and change the namespace. Is it sufficient to keep the GPL header with the files and credit the project? Alternatively, can you add another license file to the project and reference just those files in it? Would the same answer apply if using MIT instead of LGPLv3?
Edit: update based on comment received
The comment seems consistent with what I'm finding elsewhere, basically license library as GPLv3 or don't copy any parts of any project that is. So I have a follow up question now, because I have no problem using the same license, but I can't use it in all projects that I will create that uses my own library. I also have no problem writing the half dozen functions that I need from scratch, I'd just like to do what's right and still be able to write applications for my employer that they don't want to give away.
If I do use GPLv3 for my library can I link to it from another application that I create with it? The internet says absolutely not, and also yes in some cases.
I've tried to comprehend the descriptions about linking here https://en.wikipedia.org/wiki/GNU_General_Public_License#Linking_and_derived_works, but this honestly makes my head hurt.