Should I start timer immediately after adding pasta to boiling water?
You should start it as soon as you dump the pasta in the pot. I personally also turn off the heat at the same time, as you don't need your water to be boiling to cook pasta. Anything over 180 degrees Fahrenheit will do the trick. You could stick a thermometer in the water to make sure it's still hot enough, but I doubt it's possible to drop the temp by over 30 degrees unless your pasta is very cold. If you wait to start your timer and the water is still over 180 degrees, your pasta will be overcooked.
This is not true for ravioli. The pasta will get done at 180 degrees, but the filling will not. Using more water will cause the temp to drop less when you dump in the ravioli, and you should definitely keep the heat up so it starts simmering again as quickly as possible. Hopefully that's not long enough to make a difference one way or the other for your timer, as the pasta will become overcooked if it sits in hot water for a few extra minutes. I'd still start the timer when you dump the ravioli in and then check it when the timer goes off, but expect that it might take another minute to finish cooking.