That's sad about your betta!
Have you considered ember tetras? They are very pretty and completely non-nippy. They are also slow swimmers, so their movement won't stress out a betta, and way too tiny for a betta to bother harassing. Glowlights are very similar to neons in temperament. You could easily have 8 in a 55 litre with a betta. They show their best colours in a planted tank and need soft water with a neutral or acidic pH (like all tetras). I have them and love them. :)
Diamonds are like any tetra - likely to nip if they aren't in a big enough group. I wouldn't do it in something as small as a 55 litre, because as you said, bioload issues, plus, larger tetras like more room to swim than the smaller ones. They reach full maturity at around 6 months, (I think), so they grow at much the same rate as most tetras.