You can always install different versions of Python, they will coexist happily, just make sure you do not override the standard location (i.e. choose a different installation root directory).
For general use, make sure the old ones are generally not in your PATH.
When you need to test you GAE development, add the path to the 2.5 version and to GAE root directory in front of your PATH and you are set. For this to be convenient, I have created a function (could be an alias) in my
.bashrc to change my PATH variable. Works like a charm.
(echo $PATH | fgrep "$1" >/dev/null) || export PATH="$1:$PATH"
Of course locations may differ in your system.
I use the GAE variable in my PS1 so that I know when I have used that function - and I have a visual reminder to tell me I am set for GAE (i.e. I use the old Python version).