Assassin snails probably wouldn't over populate the tank however they wouldn't solve your algae problems either. A few common causes of excessive algae are - too much direct sunlight ( lights on or off) , Bulbs older than 1 year, poor water quality due to excessive amounts of ammonia/nitrate, poor water circulation ,too much leftover food or un-cycled tanks. Piranha usually don't play nice with other fishes.
