What's the best way to get Python 2.5 and 2.7
I also use the fkrull's Deadsnakes PPA. I think it is the best way to get Python 2.4, 2.5, and 2.7 at this time. I have not run into any problems using it alongside the default Python2.6. It also helps to keep things clean if you use a virtualenv, especially for those versions of Python that are not the system default.
Your best bet would probably be using debootstrap to create a chroot of an earlier Ubuntu version where 2.5 was supported, or use a virtual machine.
Installing it in Ubuntu alongside 2.6 may cause problems.