My SD grew like crazy the first 6 months I had them but the growth spurt has slowed right down. I try to see which are males and females but darned if they don't look identical. I can't tell one from another and usually fish have some sort of marking that distinguishes itself or different size or something.
Could be they are all the same sex, as well. I bought 9 Tetra's at once and never got fry from them either.
I have read it has to do with the red on there fins, mine are all the same to, as for the growth, my 1st two tripled in size in the 1st few months, then slowed down, my second 2 are looking like doing the same.