I had to go back to find the GH, and it seems to have been decided as being 14 dGH. If this is correct, I would suggest a different cory than the pygmy, which should have softer water. The Ember Tetra might have a similar issue. The cardinal tetra less so if you obtain commercial-raised fish which I believe are common in Europe (including UK I would assume). This is a 38g tank, so there is room for some of the larger species that will have less issue with the GH.
A word on 1 cm per litre or similar rules. They can be a very basic guide, but they do not apply across the board; three 1 cm fish will have less impact than one 3 cm fish in three liters of water. But even this is not really checking the boxes. First issue is the impact of fish on the biological system; many worry about fish mass to water volume, and assume that is all there is, but that is actually the least important factor. When dealing with shoaling fish, like cardinal tetras, the more there are in the group the more settled and less stressed they will be. So a group of eight will have marginally less impact than would a group of four. That's one factor. Another is swimming activity; sedate fish like cardinals do not swim actively, compared to say Zebra Danios. So from this aspect alone, one can have more cardinal tetras than one could Zebra danios, which are roughly equal in "mass," in any given tank. A third factor is the combination of species; if there is one species present that is not "compatible" in every sense with the others, the others will be "on edge" as it were, and this again impacts the system more than if the species are compatible. And "compatible" includes not just being peaceful, but how the fish interact within the species and with other species.