What livebearer fish species would be most suited to a rectangular 50 litre tank?
I reckon only guppies will fit in that tank...
It might be fun to have a dwarf puffer in that tank.
Endlers, guppies or platies
I highly suggest guppies. They are really pretty and active!
Three male guppies may be ok, however three to five male endlers would be a better fit. I would not do any females in a 13gallon.... for one they are usually bigger then the males, secondly you would quickly be overrun with fry.