It is Ni that is known for internal adaptability, and Ti and Si that are not.
Tbh, though, I don't really feel like discussing this topic.
Your definitions are too ambiguous.
I was with you right up until you started beating off to your Ni fetish again, hun. Si is adaptable as well - it's a perception function and by sheer design therefore can vary in what information it is oriented to just as Ni does. The difference is in the type of information it is pulling in.