How to stack soul gems in my inventory to make the list smaller?

You can't.

They have different values for selling, so they cannot be combined.

As already said they are different items type and cannot be combined in one lump. But if you play Skyrim on PC and you are not afraid of installing mods, there is an excellent mod called SkyUI that totally reworks how inventory is displayed and add useful features like dynamic search and so on.

Even if different soul gems cannot be combined together the result you will obtain with this mod should satisfy you enough to forgot about that problem.

You can NOT combine this into one lump as you desire.

This is because each soul has a different strength.

The ones that are stacked likely have identical souls in them, making them identical.

Whereas the rest, even though they fall in the same category, (petty) have different strengths.

The soul strength and soul gem used, cause different categories.