I've just built a 64-bit Ubuntu 9.10 workstation. Love it.
Turns out an app I want is 32-bit only. I found a handy tutorial on installing and using 32-bit libraries in the 64-bit environment.
It nearly worked but I couldn't find one specific package (libeel2-2 if that matters). After much googling and screaming (I'm a linux noob) it occurred to me to search http://packages.ubuntu.com. No hits under Karmic but it does surface in the Jaunty (9.04) repository.
So, is it safe for me to add the Jaunty repository and install the Jaunty version of this library or am I out of luck?
The dependency resolution in Debian-based systems means that if a package installs cleanly (without dependency problems) it's quite rare for there to be any major problems. It might not work properly, but it won't make a mess of anything else. Just don't force anything -- that can cause problems.
You mainly risk messing up your system due to conflicting dependencies. One dependency could end up depending on many more packages from the older version. So if you don't have many or important dependencies from the older version it should be fine. A better soulution wuold be to simply recompile the package so it picks up current packages as deps.