You need to be in on the ball, if you do it. A mature filter with low stocking in a small tank still has to cope with a sudden increase to what is I imagine closer to full capacity. It is certainly doable,but there is some risk. I have some breeding tanks, some of which are 60 to 80l and I have added fish of similar size and number, but i utilised media and some of the water from a mature filter servicing a much larger and highly stocked tank, so bacteria levels were already high. I have had no problems