They get pretty big... a friend gave us a 55g tank and it came with 2 blood parrot. I added cory's, Koi, crayfish, and snakeskin gourami.
In my case they all did well together. Down side of Koi and crayfish is that they destroy plants.
The corydoras do their thing at the bottom of the tank. I currently about 20 in the tank.
Small Koi are nice but they will get really big. SO have a plan "B" for them. Won't be over night it will happen.
I hope this helps you.