I would have said "add nothing" .........
Your triggers are not mature they are juveniles, but as your fish mature you will be playing in a new field. These triggers need space, the larger the fish, the larger the territory. As your fish grow and the space required becomes smaller, they will make space. If it takes killing one fish at a time, that's what they will do.
Many people get away with over crowding juveniles, it is when they start maturing the problems can occur. Though I agree with "dont believe everything the pet stores tell you." being said, I would also have to agree with them saying not to add these together. It is possible they are looking out for you and your tank in the long run.
Kaye