Will talking to the two secruity guards in the third mission disqualify me from getting the Ghost experience bonus?
No, you won't be disqualified from the "ghost" bonus. But you will need to hack the terminal to the door which they are standing next to. Hacking after bluffing will turn them hostile and then you WILL lose your ghost bonus.
So my advice is to bluff and perform a silent take down taking them out at once, and then hack the terminal.