Use different architecture in new independent code of a Android project to follow best practices?

So basically the scenario is this:

I’m supposed to add code and functionality to an Android project. There is one BaseActivity which all other Activities extend that provides common functionality. I know best practice according to Google is to have one Base Activity and multiple Fragments because those are “cheaper” than many Activities. My deadline does not allow me to make fundamental changes to the existing code and quite frankly it is hard to get a grasp of how everything works together since in my opinion the code is not very good. My code is mostly independent from the existing code so I wondered whether or not I should create my own Activity including multiple Fragments or follow the current architecture to not confuse future devs that have to work with the code.

Would you people recommend me?