Yes sit flush, I make sure there is no water bubbles in them they tend to sit tail high. They don't seem to move very well in a circle going forward.
Ideally a spearing decoy should be nose heavy when placed in the water and allowed to sink at it own rate. (no tension on the line) If the decoy is sinking with the nose down, it should be moving in a forward direction. With a wood decoy, the nose heavy condition is controlled by where and how much lead is added to the decoy to offset the buoyancy of the wood body. I assume a plastic decoy has this determined when the mold is made.
Ideally a spearing decoy should hang level when suspended in the water. Again with a wood decoy, this is controlled by the placement of the screw eye the line is tied to. Should hang level in the water and hang nose high out of the water. I would think the placement of the hanging eye would control the level of a plastic decoy as well.
One could glue a single or multiple lead shot to the bottom of the decoy in the appropriate places to alter the balance. Many guys use lead shot from a shotgun shell to weight and balance wooden decoys.
One could