if the py2.5 script is relatively stable, meaning you don't need to change it much or often, try turning it into a self contained executable and call that instead. There are two python-to-exe programs I see mentioned often: [py2exe][1] and [pyinstaller][2]. I've personally had the best luck with pyinstaller, though I never used it in a gis-inside-gis fashion such as you are attempting. There are some other py-to-exe's discussed [here on Stack Overflow][3].

However responding to the problem which is driving you to use 2.5: I used python 2.6 with Arcgis 9.3 for a long time to good effect using the recipe at https://gis.stackexchange.com/questions/2226/can-i-use-python-2-6-with-arcgis-9-3.

  [1]: http://www.py2exe.org
  [2]: http://www.pyinstaller.org/
  [3]: https://stackoverflow.com/questions/2136837/process-to-convert-simple-python-script-into-windows-executable