what i mean by that is:

remember in oblivion how the potency of a soul was represented by numbers? petty souls were a hundred something, and grand were a thousand something in worth.

its the same thing for skyrim only it doesnt tell us its numbered value. they should be able to stack to fill jems up completely.

Example: a couple petty fills a lesser, a couple lesser fills a greater. a greater plus a bunch of petty fills a grand. a whole LOT of petty fills a grand. and so on.

im tired of having more than a few greater and grand soul jems filled with lesser or petty souls. its a complete waste

To empty a gem, throw the Soul Gem on the ground and pick it up again, then it will be emptied. This can clear out a low level soul from a high level gem.

As for the smaller souls, it is actually better to have them than Grand Souls for purposes of Re-Charging an Enchantment. You get points towards increasing the Enchantment skill with each re-charge, not how much you charge up each time. To that end, I only use Grand Souls for making things, and only smaller souls for recharging.