I would observe them for a while before buying more.
My suggestion would be to check for shell guarding behaviors for a couple more weeks.
What kind of substrate do you have in the tank?
At what temperature do you keep the tank?
Are you absolutely positive you have both genders, and not just that the store said so?
How often do you change the water?
What are your readings for ammonia, nitrite, and nitrate? (Answer in PPM, please?)
How much larger than the fish are the shells?
What kind of lighting schedule do you use?