Thank you so much for your suggestions @pod
! I would love to try the sphagnum idea! Yes, I have been filling the bigger ones with some of the smaller, but the problem is that when doing so, the surface of the soil that the big, "main" plant is in, stays moist for ever under the smaller pots, even if they are for example little nursery pots with succulents. So, that is nice for some of the plants that love moisture, but not to the ones that hate it! The sphagnum would solve this, as it would absorb moisture instead retaining it into the soil.