try to get close to a ceiling rafter and when cutting through old plaster i find running the hole saw in reverse there is less chance that the plaster will fall down.if plaster is really bad and thin i will pencel out a pancake box score it with an exato knife untill plaster falls away from wooden laths and use inch 10 wood screws to fasten to ceiling.As for running from switch to light try to find out which way ceiling joist are running ,notch out in corner of ceiling and wall fish down wall into basement and up to switch location. good luck.
