Hi Videl,
For a 12 gallon tank i would go with 6-7 cardinals and 3-4 pygmy tetras or amano shrimp. That way you have something of interest at the bottom of the tank also.
Is the tank already cycled? Cardinals can be a bit delicate if added to a tank that isn't fully cycled.
There isn't a lot of room for an algae eater but you could add an apple snail (these come in all kinds of colours) or failing that the shrimp would eat some algae.
As for the rest of the tank i would have a sand bottom (this is good for the corys to root around in) then i would have some bog wood with plants growing on it (jave fern, Java moss, anubis nana). These are all plants that will grow in low / medium light without problems.
Hope this helps
Emma