Old plaster walls... I feel your pain. Ours are essentially concrete on lath, brutal on drill bits (have to use a masonry bit to get through the plaster, then a wood bit if I hit lath behind that) and an absolute PITA to find studs in. I cannot begin to express how much I HATE trying to find studs behind old plaster. Seems like the stud finder (I've tried 3 different models over the years) always looses it's depth reading and either starts picking up the lath:
S.F.: "It's solid wood everywhere! Beeeeeep Beeeeeep!
Me: "You stupid piece of useless plastic, I'm going to get a hammer and just start tapping everywhere"
or else it starts reading everything as void:
S.F.: "There's nothing there! OMG, the whole wall is empty! Lookout, the house is going to collapse at any second!!!!"
Me: ::mumble mumble:: "melt this freaking thing down... "::mumble mumble::
My most recent tactic (don't try this at home folks, it's an exercise in sheerest frustration) has been to take lots and lots of readings until the ceiling/wall is crisscrossed with conflicting outlines of beam/studs that can't possibly be going the directions that their outlines suggest, find the greatest concentration of tick marks, thump it for good measure to see if it's hollow and then drill the smallest hole I can with a masonry bit and *hope* that I hit wood. Once I've got one success I'll measure from there, and if that doesn't work, I'll give up and just start drilling test holes where I WANT the dang beam/stud to be. I generally find the beam/stud a LOT faster once I give up on the stud finder and just start drill *ahem* "exploratory holes".
Oh yes... Before I started this diatribe on the vagueries of Stud finders when presented with anything other than nice easy dry wall I was actually intending to answer the questions... Lets see...
1.
Do you do any other type of support other than just 2 screws to each stud?
I believe the spec is that you only need to hit two studs... NOT two screws in EACH stud... Good lord I would have gone stark staring mad... And Susan would have been ready to send me to local sanitarium. Honestly I just went down the rail putting lots of screws in maybe one every inch or two, if I was able to locate an actual stud I'd work from there by measuring and drilling (as you mentioned above) and if I missed? ::shrug:: put one in on either side and call it good.
. I hit with several screws (nice solid square drive 2 1/2 inch deck screws) and that rail was ROCK SOLID. I'd suggest putting one in where you think your first stud is closest to an end of the rail, rest a level on the rail and move the free end of the rail up and down until it's dead level then sink another screw in at the other end, if you hit a stud, great, if not? ::shrug:: those walls are plenty strong enough to hold just the rail in place and dead level while you start hunting for studs by sinking screws... Make it a game! "B-4" Hit! "B-5" Miss! A-4 "You sank my battleship!"
2.
My ceiling is about 8'. I plan to put my 39" wall cabinets flush with the ceiling, so I should draw a line for rail 2 1/2" below the ceiling, right? Sit the rail right above the line, and screw into studs (if there is one!). Should I leave 2 3/4 just in case the ceiling is not flat? (I used alevel for the lines, but I don't quite trust myself).
I don't trust walls/ceilings to be plumb or level at any time, in any room. That said, I'd go ahead and draw your line at 2 1/2 (which leaves you about 1/4" of wiggle room) and hang your rail as described above but with just the first 2 screws, one near each end. Check the clearance by running a scale from side to side on the rail and see if you're going to have any problems. If it looks iffy, take the two screws out and drop the rail a bit. You have a fair amount of up/down play when you're actually hanging the cabinets on the rail so you'll probably be fine.
Now that that's out of the way, I do feel obligated to point out the NICE features of our PITA concrete like plaster walls/ceilings. Good sound deadening. Excellent thermal mass. It doesn't provide insulation so much as thermal mass to decrease the temperature swings from day to night... Our house stays cooler in the day and warmer in the night as a result... Though I wouldn't mind some actual insulation thrown in, and plan on insulating the floors someday. ::sigh::
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can be a stand-up comedian! Well, he probably did it 'sitting down'. 
