Yes, as long as you don't link in their libraries or incorporate other parts of their code, you should be able to license your extensions and stuff that ties into some APIs as you like.

Companies usually like permissively licensed projects like MIT, Apache or BSD. But if you want them not just to take your work, a copyleft license like one of the GPL licenses is a good choice.

You can reasonably expect that configuration data would be completely separate from the app it is configuring, and could release it under any license. The only way I can see that their license would carry over would be if you were shipping parts of their codebase with your customizations.

Think about it this way: if what you are shipping is just an arbitrary stream of data that contains no proprietary work, then it stands alone.

Of course IANAL and there maybe some ridiculous law that covers configuration data, but I'm not aware of such a thing. (Waiting for people to pull out all the legal precedent that I am missing, Cunningham law and all.)

The employer doesn’t claim any intellectual property rights over my work product. I’m not able to find anywhere that the proprietary vendor does either.

You're probably in the clear. Legalese isn't so opaque that you would miss a section about this.

Of course, that doesn't stop them from suing you if they decide your work could be very profitable for them.

GPL effects "derived works". So if your code is derived from proprietary code, you can not use GPL, as you would need to re-license the proprietary code and you can't do that (assuming you do not hold the copyright for the proprietary code). LGPL and permissive licenses are probably fine though.

Now what exactly is a "derived work"? That is unfortunate up to interpretation and different organizations draw the line in slightly different places. We'd need people to go to court to get that line nailed down more firmly.

What are the considerations here? Can I use the GPL or is it inherently out of compliance since you need a proprietary software to run it?

While its often overshadowed by the giants like MIT and GPL there are times when MPL is appropriate.

Its copyleft at the file level and allows them to do what they want with the rest of their project, this may suit your needs.

This is a pretty reasonable rundown of the pros and cons:

https://fossa.com/blog/open-source-software-licenses-101-mozilla-public-license-2-0/

First off, IANAL. Not legal advice. Consult with a lawyer or at very least some place like the Software Freedom Conservancy if you want legal advice.

If you've got an employer who is willing to let you own the copyright on stuff you write as part of your job, that's pretty awesome. But if you don't have that in writing, it's probably still risky to move forward under the assumption that you own the copyright. If you haven't specifically had a conversation with your employer about this, I'd definitely say you need to before you conclude you're in the clear on that score.

But assuming you've got all of those i's dotted and t's crossed, the next question is "where do you live?" If you're in the U.S. this court case is probably relevant and (IANAL!) probably weighs in your favor.