I believe your first question means whether or not a properly built and balanced airplane can glide without power, and the answer is yes. Obviously the flight pattern without power very much depends on the air frame, but the answer is yes. If you mean whether or not the plane will remain airborne after "throwing" it - and assuming you are talking about a real RC plane with all the gears built in it - you would find it to be a very short flight! An RC plane with all the gears in it is relatively heavy and will need a certain air speed to remain airborne. You may not be able to impart the proper air speed to it by throwing it, which means the story will not have a happy ending. I am also assuming you are asking this question for educational purposes and not really intending on trying this. If so, you may want to consider free flight planes that are designed for this purpose.

As for your second question, unless you already have an engine that you want to find a plane for, my recommendation would be to choose your plane first, and then match the powertrain to it.