I agree with bonzo. If you are looking into breeding you want babies from different spawns. Also, if you make the decicion to breed jack dempseys, bear in mind they spawn upwards of 300-500 eggs, and are hard to sell as they get big.
I would think that getting rid of all but 5 would do and you should get a pair out of them. Changes are very slim that all 5 would end up being the same gender. Moving them on as they are smaller is definitely easier than when they are larger.