64 liters is 16.9 gallons.
You don't "need" an airstone if the filter is creating enough of a disturbance for good gas exchange. With plants you don't want to outgas all the CO2
in the water.
It creates an updraft current in the water and the water surface gets disturbed which is good for gas exchange.
I also agree with max that the extra guppies will be fine, and I'd skip the shark.