Do I write narkotykow or leki for the word drugs as in'I sold people drugs'?
narkotyki. Leki are for healing, narkotyki for poisoning (to make it simple).
Also: prochy, dragi.
'miałem seks z kobieta' is that ok?
Very unnatural.
Przeleciałem twoją kobietę - it's slang, but not very vulgar.
Lots of other variants are possible, depending on how vulgar you want to get.
Finally 'gangstas'.
Gangstas are gangstas - a purely American breed :)