I'd up the corys to 6, the serpaes to 6, and depending on what type of pleco rehome it. If it's a "common" pleco it will get huge and will be stunted in a tank that small. If it's a "bristle nose" or "bushy nose" 20 gallons is bare minimum, 30 gallons is better. If it's a clown pleco 20 gallons is fine. I'm guessing you don't have a more rare pleco like a L134 so I won't get into that.