Close to the bottom of this website it will answer your questions.
Yes, you can have different record lengths or parameters for different versions.
Although it’s <i>possible</i>, it is extremely not recommended. If you length of a generation doesn’t match the model, you’ll either be adding all subsequent generations as being different from the model (and there fore needing to override each one) or you’ll have an in compatible generation.
It is much better to start a new GDG with an appropriate model and make a clean break.