The dynamics of Interaction of GPL code with proprietary code has to be looked into for derived works as it differs on a case-to-case basis.
As per GPLv3, You MUST include Install Instructions and the user should have the capability to recreate the same. This condition might lead to providing proprietary information as well.
Any modifications to or software ...
This depends on exactly what they've said. If it's
We'd rather you didn't use non-stable libraries with this product. If you do this, we won't help you.
that's absolutely fine, and very similar to the position the Linux kernel community take with non-open source modules in the kernel. Even something like
We'd rather you didn't use non-stable libraries ...