Not being sarcastic.....(maybe a little)
But you can have any plant you want in a 20 gallon. Is it a 20H or a 20 long? How much light? Do you inject CO2? What kind of substrate are you using? How do you dose Micros and Macros?
Sorry...just trying to cover all the bases at once. The crayfish will most likely shred most every plant in there unless its a real tough plant though.