Ours came in super small. Maybe that was the difference.
Sometimes it's a game of chance. One person's clown may be safe with their dwarf and in another person's tank it's lunch. Not only does it depend on personality, but it largely will depend on hunger. If you leave them lean and mean, they may take down a smaller fish that once was safe.
We had a Ghoul and a Barnacle Blenny together for a long time. That wasn't a planned union, but we just could not get him out. We had acquired an amazing ghoul and he needed to go in a stable tank pronto. So all but the one went in with the orange spot filled fish..... he got left behind. I could have caught him a few times after that, but he wasn't going in my OSFF tank after being in contact with the Ghoul. This was basically turned into the Ghoul's QT. Anyway... I digress. I'm sorry if anyone has a big attachment to Barnacle Blennies. Then after a few months, he was gone and the Ghoul was happy. :-(
And THEN, not only to do have to deal with differing personalities throughout the species, you also have to worry about changes in personality of the one specimen. As they get older, they change....
Then you have to worry about size potential verses reality. You can get monster adults at the fish store sometimes. If I grew one up from a little small baby, it will not reach it's full potential.... at all. I believe that to be related to diet. So, given either situation, either may or may not be appropriate.