If your application needs to add custom validators, then these are easy to develop. In order to keep your application portable, it is best to add these to your application's modules folder rather than amend the base validators.py
The new validators.py file needs the same header as the original (to import the correct modules) - just change 'storage' to 'gluon.storage'.
Then at the top of your model's db.py add: exec('from applications.%s.modules.validators import *' % request.application)