Blue Acaras are no more aggressive than keyhole cichlids, angelfish or discus when breeding. They have their territory and will chase fish out of it when they have eggs or fry. But they won't go out of their way to kill everything in the tank like some cichlids do.
You will notice a change in the other fish if the acaras become really agro. The other fish will act nervous and skittish and hide in the opposite corner of the tank (acaras on one side and all the other fish on the other side). If you see this happening, then you will need to move the other fish out. But blue acaras don't normally do that unless the tank is small, (less than 3 foot long).
You can use plastic storage containers as rearing or emergency holding tanks. Put a thin layer of gravel on the bottom of the container, half fill it with tank water, put an established air operated sponge filter in it, add a heater and fish, top up with clean dechlorinated water. Cut a hole about 4 inches in diameter in the lid so you can feed them, and then put the lid on the storage container, and away you go.