I built section (top plate+ studs+ bottom plate) on the floor and raised the section up into the space.
It did fit into the space, but the floor is sloped, so one side of bottom plate sits on the slab but opposite side is 1"1/2 floating from the slab.
(tilting side to side. not back and forth)
Is this OK with adding shims or spacer? I am going to use Ramset(gunpowder nailer) to secure the bottom plate to the floor.
Or, should I cut each studs in different length so the bottom plate goes along with the slope?
Not ideal, but not the end of the world. Since the wall is load bearing you can get away with it. I would keep the bottom plate flush with the concrete so you can shoot it in good with the power shot.
With the top plate, I would take another 2x4 and run in either on a table saw or with you skillsaw to make a wedge to fit your gap. Attach the wedge to the top plate and then attach the top plates to the joist. Once the sheetrock is on, you'll never know.