I wouldn't mix more than two together. I have to say i really regret mixing my pfr and crs in my 10 gallon. They breed so fast! i would have liked it better if i had them in separate tanks, so i can have more of them. But too late now, i don't want to stress them by moving them.
You can mix crs with neos, if you are careful and squeeze just right in between their parameters, but like i said i regret doing it.
You can do neos mix with malawa shrimp, they like similar parameters, i think.