Hmm, actually all this prooves is that these two males have lived together for now (I think the pale coloured one is male too).
It's not proof in any sense.
I agree with Betta that it may be that the more developed male doesn't see the younger one as enough of a threat yet to merit a full on attack. So although I'd say this prooves that agression levels aren't as high in yours (as most males will attack females aswell), you can't make a general statement.
Apart from anything else, I've seen at least 3 members in the last month who've had males jumping from their compartment and into a neighbours. They've then been ripping eachother to bits before being seperated.
So yes, I think it's entirely possible that the agression could return to a more natural level over the generations, but we're defintiely not there yet.