I don't like internal / external since I can see N,F / T, S in that way also, it would need more specification. N, F information may be externally derived(E) but internally directed(what's hidden, unspoken); the same for T,S except it's externally directed(what's apparent, obvious etc).
I could be wrong but it seems to me that you talk of functions as if they were strictly Ni-Fi and Te-Se. I'm sensing a bias. And yes, I'm positive you are an Ni-Se type and slightly certain that you are a Te-Fi type.
Assimilation is about serving the dominant. Both are about supplementing the ego. Just one fits into the environment, and the other fits the environment into itself. "
What is the ego? Is it the dom function? How exactly does assimilation or adoption serve the ego? Why does Fe need Ti to justify itself? How?