There's no general answer; in many cases programs made for Debian 7 will work for Debian 8, although you'll often need to install supporting packages from Debian 7.
In Skype's case, you should install the Skype alpha anyway if the people you want to communicate with are using Skype on Windows or macOS, since the old Linux client can no longer communicate with current versions of Skype on these platforms. The Skype alpha is available for Debian 8; simply download the
.deb and install it using
sudo apt-get update
sudo apt-get install gdebi
sudo gdebi skypeforlinux-64-alpha.deb
You'll find detailed instructions regarding all versions of Skype on Debian 7, 8 and 9 on the Debian wiki.