Is there no posledny with hard "y" (sorry no accents on keyboard)
No, poslední follows the soft declination (you can check the Tips and notes).
is there a difference between using "Stroj" or "mašina"
If I were to say "Nejste Stroj" to a woman, would that be correct or would I have to use "Mašina" because it is feminine?
It would be correct. The grammatical gender has no connection to any physical or other gender of the person. Mašina is quite colloquial.