Mat,
I have several pony cylinders. I have measured the buoyancy of each one in a tub of water using a string and scale.
Generally, when rigged with a regulator and sling, a full aluminum pony cylinder will be several pounds negative. HP steels will be significantly negative.
I would recommend redistributing weight from one side to another so that you are in proper trim. Whether you wish to also remove some weight from your belt is a personal choice based on your assessment of the relative risks of an uncontrolled ascent if the cylinder is lost, vs. the inherent risks and hassle of overweighting.