The size advantage may not help the capa but it makes this gamble less dangerous. Still, overall it is pretty dangerous. Gotta be prepared to separate but it sounds you won't have the right tank/pond anyway...That’s sad news mate. I shall keep an eye on them. I’m abit tight on space, untill everything’s moved. that capa is a lot larger around 19/20” so I’m guessing for some time they shouldn’t be okay, valiant around 11”.
My capa from 22" to 27" had been housed with a 12" tig and it was biting its fins, so the tig had to hide. I wonder if your capa will show aggression towards the piraiba and the vaillantii, instead of vice versa.... and what will come of it.
I will preach what I don't practice Don't buy fish until you have the right tank with the right tank mates. Most definitely this goes for capa and rosseauxii. Just like me, you will regret it sooner or later, even though hope floats.
$0.02