I agree with pcnicholson, my platies are the entertainment of the tank. They are colorful, smart, and so different from each other. Of course they don't school but I have a pair(m/f) of red wags and they are like husband and wife. They do everything together! My other two females love the snails and plants in the tank. They play with them all day it seems.
Since you're alittle limited on adding more fish I would recommend 4 platies. If you don't want fry get all of one sex or just get one female and 3 males.
If you're interested in something else I would go the with only one pair instead of both. IMO