Hi Oscar, Your not thinking of keeping an octo with a picasso trigger are you? I hope not, because the octo would probably become food for the trigger. I had a 10" Picasso in my reef for some time. He ate all shrimp, crabs and snails in the tank and was quite unruly. Didn't bother the corals, though. He was just way to aggressive at feeding time. I also have a large zebra moray eel in the tank that I hand feed and "Wilson", (my trigger) was making it nearly impossible to feed him. Attacking my hand and stealing the food before the rather slow eel would have a chance to get it. He was a great fish loaded with personality but I would never put another in my reef. I sold him and he's now in a 180g fish only tank at a doctors house, doing well last I heard. If you must have a trigger, check into the "blue jaw" trigger. They're very layed back and a lot of fun.
It really depends on which trigger you get. Pink Tails, Blue Jaws and Sargassism Triggers are considered pretty reef safe. My blue jaw is a sweetheart. I don't think I'd add any of the more aggressive triggers like a clown or undulated. Keep the trigger smaller than the current fish in the tank. My Picasso didn't pick on any of the other fish, just inverts. He did try to eat a rather large blue spotted watchman as I released it from the bag once. Had the goby in his mouth up to the gills and then spit him out. That was one lucky goby.
Niger Triggers are one of the less aggressive types also. Forgot to include it above. I had one a couple years ago when I was first starting my marine tank. He stayed hid a lot in the rock. A 2" Picasso would most likely do well. Just remember that a 2" trigger will quickly grow into a 6" trigger and then up to 10" or so. Just be careful adding any new fish when it starts to get larger!
Triggers are voracious eaters so they grow pretty fast., At least the ones I've had, did. I added a yellow tang to my tank that already has a red sea sailfin tang and a blond naso, and he was initially pick on by the sailfin, But they are the same species, Zebrasoma. This lasted about a week, but now all are happy. The naso didn't bother the yellow at all. I think you could add it ok. (JMO)
Oscar, You'll have better luck if the more aggressive fish is added last, in most cases. That's not to say that when it grows and becomes comfortable it might not become the tank boss, but it will usually tolerate the fish it grew up with.