Well, I am not a native English speaker, and I don't know what native English speaker find normal but my dictionaries give kilo as a legal word for both kilogram and kilometer. Usually it will be clear from the context which of the two is meant, and of course you can always use the full word. Wiki.answers.com has several questions of the form 'How many cups in 1 kilo' and ' 'How many pounds is xx kilos'. I guess that in formal language the full words are used but it seems correct English to use kilo.
Can't talk for native English speakers either, although I never heard saying "kilo" when "kilometer" (or kiloliter, or kilobyte, etc.) was meant.
In Portuguese it is clear that if someone says "quilo" they mean "quilograma/kilogram". In all the other names - "quilômetro", "quilolitro", etc. - the full word is always used and a short name doesn't exist.
Correct. So don't use it in scientific literature. However in standard vernacular, where "math" is an accepted substitute for mathematics and "cell" is accepted for cellular telephone, kilo is perfectly fine to use to refer to kilogram, especially since there is no other SI unit that is ever referred to in this way in common language.