Do notes from non-natural minor scales fit well in the associated
They can, depending on context.
I wondered if this means that F# and G# would be the "least
controversial" notes to add when playing in C major since they come
from variants of the associated minor key? i.e. they'd fit quite
nicely without sounding too dissonant?
Are they somehow the least controversial, or least dissonant? No. In fact, this question doesn't really make sense when you consider the meaning of dissonance.
The fallacy in your assumption is in thinking that notes outside of a given key somehow have a certain amount of "dissonance" inherent to them by virtue of their being outside of the key. This is not the case. A note's dissonance can only be measured against other notes sounding at the same time. Simply put, if there is another note sounding that is a second, seventh, or a tritone away, then there is a dissonance. If it's a third, sixth, perfect fourth, or perfect fifth, then it's consonant.
Therefore, to determine whether a note is dissonant (or "controversial") depends entirely on the other notes in the chord that is being played. In fact, by using the correct chords, any note outside of the key can be made to sound quite consonant. The correct question is in what context (i.e. in what chord) that note belongs to make it consonant, and then what implications that chord has on the chord progression.
In the case of G# in the C major scale, as others have already mentioned, it creates a consonance with E and B to form an E major chord. The result of raising the G to make this chord major is that it acts like a dominant chord, strongly implying an A minor tonality.