I have my large aggressive plakat in a ten gallon. I recently took some bad advice from an LFS (...sods) and ended up with two very small loaches in there as well.
Cole occupies every inch of that tank, patrols it all day, and I should have known better than to think he'd tolerate even peaceful fish in his territory (even if they weren't an entirely stupid choice for that tank). He's killed one already, and as I can't catch the other he hunts it all over the tank, flaring like a maniac.
I honestly feel bad only giving him ten gallons - and do think a planted 20 long would suit him better.
I feel -really- sorry for any fish of his size and disposition stuck in a 1 gallon bowl..