Too many fish (especially if you add plenty at one shot) will mess up your tank's chemistry, and likely result in casualties. I think giving the fish you have (although I won't say your tank is overcrowded) the space to grow (cardinals can reach a nice 1.5 inches) will be much better than finding new fish to "fill" the space.
Actually, aren't cardinals red enough already? Is your dad trying to incorporate some "lucky" element in your tank or something???
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