Officially no: these are too much of a moving target.
In practice, since Forky (testing) has been branched from Trixie (13) quite recently, current Debian-13 repos will most likely work fine on Forky for now.
Yes, this is the general idea. For example Bookworm (12) repos do not support glibc-2.41, so if you use them on D-13, you will run to issues with header files.