I am looking for confirmation on some of the info I have found and answers to a few missing pieces. This is regarding wire size and conduit size for service conductors from the meter to the breaker panel (about 30 ft away).

Wire Size:
125A service needs #2 copper or 1/0 aluminum -- #6 copper ground.
150A service needs #1 copper or 2/0 aluminum -- #6 copper ground.
200A service needs 2/0 copper or 4/0 aluminum -- #4 copper ground.

Is this correct? These would each be 3 wire services plus ground. Do all 3 wires have to be the same size or can the neutral be smaller? If so, how much smaller?

How deep does PVC conduit for this run need to be (no driveways involved)?
What diameter PVC conduit is needed for each of the services above?
Specifically I am looking at either 125A or 150A service and would like to use a conduit big enough so that someday in the future I could run another set of wire back through this conduit that would provide photovoltaic current back to an REC meter (and back to the grid). Is 2.5 inch conduit adequate for what I want now and in the future?