Yes you can reuse the old pipe and reduce the size at the pump.
What was being used to support the old pipe & pump? A well seal? You can reuse that also and bring the wire through one of the smaller ports in the seal.
No you don't need a pressure tank, but you do need a way to control when the pump goes on & off. You could use a float switch in the big tank wired in to the 3 phase starter to accomplish that.
Submersible pumps absolutely can have back pressure on them.