# when you mix repositories usually it is quite smooth going,
# but the problems are with apt package itself - since almost every repository
# contains apt, every repo will try to overwrite your apt with its own.
# so we decide to pin apt to specific repo (we only use apt from sepcific
# repositorium - CCRMA here), /etc/apt/preferences:

Package: apt
Pin: release c=CCRMA
Pin-Priority: 1000

Package: synaptic
Pin: release c=CCRMA
Pin-Priority: 1000

# this pins apt and synaptic... with such setup you should have fine running apt installation...