one site mentioned they are egg layers and the eggs are prone to fungus... In hard water am I correct in assuming this isn't a concern?
Indeed. Characiforms are not so easy to breed, and the young are hard to raise. Don't worry about getting flooded with baby congo tetras (and if you did you could sell them easily). If you did want to breed them, then soft water would become more important. The syno would certainly eat the eggs anyway.
Also eyeing the Buenos Aires Tetras.
These have a reputation as fin nippers, but whether they would go for a syno, I don't know. Generally the bigger the group, the less likely they are to interfere with other species.
I found a couple smaller cichlids (4.5 - 5") that are supposedly not aggressive: the Sunshine Peacock and Yellow Lab. Considering maybe one of each instead of a school of small fish. Only they live up to 10 years
These cichlids are not going to do so well as singletons I don't think. They have complex social interactions and need to be kept in groups.
P.S. Congo tetras will live 10 years too, and probably the Buenos Aires tetras as well.