If you don't mind, can we stick with my original question, whether it
is OK or not to put some landscape rocks on the surface of a pot to
make it heavier?

An inch or two wouldn't be a problem, I don't think. That's the kind
of depth (roughly) that you might put in for mulching purposes.

A really thick layer of rocks might tend to compact the soil, though.

What we do here is to put bricks on the pots (well, or on the trays,
I'd have to go out and look exactly where the bricks are and whether
they are supporting the side of the pots or on top of the pots). That
way they don't affect the soil.