Well, I can only speak from my experience. We got our Pinktail (my fave) 4-1/2" to 5" a year ago. We also got our Picasso at that time 3" to 3-1/2". The Pink had been in two residential tanks and brought back to the lfs. He was in a 55 gal. with a shark 14", a puffer 10", and a couple other fish with no hiding spots. We put the Pink and Picasso in at the same time and they got along beautifully. No show of aggression from either. He is the coolest in my opinion. (My man likes the Undy) And I love our Picasso, he just doesn't give a damn about anything. He mind his own business, until the Undy gets spazstic.
Then we got our Undy a couple months later, 4" to 4-1/2", she is not like other Undys you have read about, we've had her almost 7 months or so. She is a shy little girl. The Pinktail dominates her, they got into a couple rowls the first week then, the Pink was the boss and remains so to this day. No aggression other than his change of colors (which is sooooooooo cooooooooool
.
The reason this is long, is that they are all happy fish now. As a matter of fact, we think the Pinktail is in love with the Undy. He follows her around, lays flat on top of her, and they both just hover there for a couple minutes, then they swim to another area and repeat as above.
Our Pink is famous for moving HUGE pieces of live rock. STRONG he is.
The reason I am going into detail, is that ours was tank hardy, or tank prepped, so he may act differently than one who has just been caught and tanked. Sorry this is long, and boring, but I hope it helps.
I guess, in short, they are listed as less aggressive, ours is not aggressive toward the other two or the shrooms or crabs, so............Peace