yes size does matter you want to keep them roughly the same size because when you add it they will size each other up. also rearrange your tank decorations evey time you add a new fish because triggers are very territorial.what do you mean by clown picasso? if you mean clown trigger,yes. also keep in mind some triggers you can choose from are timid.for example the blue throat trigger.we have one and we can't keep it with our other triggers because he gets beat up to bad.he hides all the time to.he is a very passive fish.