In joking, and off frequency, we will say "maintain TCAS seperation with", but having an a'c on TCAS does nothing for us. Until you actually have the aircraft in sight and have been told to maintain visual seperation with, we will use another form of seperation. Sorry, but having him on TCAS aint worth squat.
As for controller passing tfc that is 1000' above or below and will converge or be very close, we are required to. It is normally a preventive measure so that when you actually do look out the window, see this other airplane that 'looks' like he's at the same altitude, and aimed at you, you aren't a little shocked because you've already been informed of the tfc.
Yes, if you are IMC, and therefore won't be able to get an a'c visually, let us know. As you are aware, we don't have windows and therefore can't tell where you are in relation to any clouds.