Smart gameplan!
Though there is one thing you haven't seen yet to know - past level 3, the leopard actually needs more food to level than the other single element critters. I got mine to level 9 before breeding her and the food cost (total food, as in the number on the screen times five), instead of 5, 20, 40, 80, 160, 225, 350, 500, 650, the leopard costs 5, 20, 40, 100, 250, 500, 800, 1100, 1500. So a total of 4,315 to get from 1 to 10 as opposed to the 2,030 the other commons cost. But at level 9 she makes 316 cph.