After my Colombians spawned I had 25 of them (4 adults and 21 fry) in a 40g tank.
They're not much on schooling, though. I moved them all into my 125g tank and some of the juvis will school with the Hengeli Rasboras, but most are just scattered throughout the tank.
I saw a photo of a large tank (maybe one of Jeff Senske's) that had hundreds of Bleeding Hearts in it. It looked awesome.
For any of the bigger tetras you could do 10-12 in your 55 so long as there's not a lot of other fish already in there.