The correct changing that you want would be:

" Homo sum; humani a me totum alienum puto. " = I am a human being; everything human is strange to me.

(TOTUM HUMANUM ALIENUM A ME PUTO would also be correct, but in formal Latin that other order is more classical = HUMANI A ME TOTUM... < direct object TOTUM put after indirect object A ME and also this being put after that genitive/possessive HUMANI = [TOTUM] " OF what is human " > humani a me totum..., and the characteristic given to the direct object, ALIENUM, thus neutral too, as not a verbal complemente in fact but connected to it, referring to the direct object, put by the end of the sentence, just before but close to the verb, PUTO = I judge / consider). In a direct order it would be " Sum homo; puto totum humani alienum a me = I am a human being; I consider everything [that is] of human (species?) [as] strange to me!!!

TOTUM is opposite to NIHIL, both neutral forms (from TOTUS, masculine, TOTA, feminine = every, all; NIHIL [also: NIL, or NIHILUM, far less used], just neutral = nothing).
I dare to stress that "omnis" would fit better. Remember "Gallia est omnis divisa in partes tres"? Omnis, because it was not a unity but a variety.

I have one more variant of this proverb :