I have a 75 gallon that I house crays and cherry shrimp in. The crays will eat the shrimp if they can catch em, I've seen it happen... I would suggest you upgrade your 20 gallon to a 55 for the crays. They can be a little territorial. You want to have space for them. Also, they can get to a decent size... 3"+, but takes them a good year to reach that size.
Ok. If I have any luck with them I will put them into a 40 breeder ( I hate tall tanks

