I don't know the situation on Gentoo, but partial updates are explicitly unsupported on Arch, not least because they don't do stable ABIs; Debian should have a much easier time upgrading just one package.

It's not necessarily recommended to mix stable and testing but it mostly works fine in my experience. I'd guess Gentoo gets around quite a few problems as everything is compiled from source. So updating a single libary would cause a rebuild of everything that depends on it.

Gentoo also has the concept of "Slots", so you could have multiple versions of the same libary installed and packages will choose their version to build against accordingly.

Having used Arch and Debian, I‘ve definitely had an easier time installing the latest version of arbitrary packages on Arch. Something like SDL is part of base and thus is already running latest.