depends on species but at .5" you can mix pretty much any of them. I don't recommend it however unless they are very different looking and you plan on separating in the near future. Reason being as that size they wont bother each other but if they are similar looking species (peacocks especially) then they will be hard to distinguish from one another and you wont know what type they are to separate them. If dealing with fish that are more aggresive then the larger they get the more problems you will see.