Pygmy puffers, aka dwarf puffers are completely freshwater and do not require any salt.
if its 15 litres, I'd argue its too small to even hold 2 dwarf puffers, and I'd definatly not add anything else. If its a 15G long, I didnt say anything.
Out of experience, I can tell that corydoras and puffers do not go together. The puffers seem to even go out of their way to get the corydoras and nip their fins. I dont know about kuhli loaches, but I can definatly not recommend it. Chances of them ending up dead within 4 months are high.