Yes, definitely get fish from the same batch. As most of the BNs aren't 100% identified by species, if you get fish from different sources, they aren't necessarily the same species (and most likely won't breed). I think they're able to breed at about 3".
With the cory, I'd keep treating the main tank. If one fish caught it, it's better to eradicate the disease from the whole tank in case others catch it.