i hear adding cariba to a red belly tank will often bring out aggresion in the red bellies... so it makes sence that reds added to a cariba tank would not be skittish... but this is only what i've heard/read here... no personal experience!
i have noticed adding more p's to an existing tank helps anyways... i had 2 rb's for a while, then added 3 more... i find them much more aggressive now... but then again, it may have been their age/size, or the fact that their tank size was more then doubled when i did this?!?