For example : "She looks super slightly similar." would mean she looks a really small bit similar. From a french background, we allow ourselves to combine certain superlatives in this way, but I cannot find anywhere with such usage in english.
No. These Google Ngrams strongly suggest that 'super slightly' is non-idiomatic, with which indication I'd agree. We'd use
She looks ever-so-slightly taller.
[Hyphens optional IMO] [and I've switched to the 'taller' example]
(57 000 Google hits for "ever so slightly taller"; 7 dubious hits for "super slightly taller")
ever so (phrase)
‘I am ever so grateful’)
[no hyphens when used predicatively]
And an adverb-modifying example from Collins:
I saw him pause ever so slightly.