From experience you can try the swap and throw away, but I can guarantee you the birds will know that it is a fake egg the more you do it. Eventually they will push the fake eggs out the nest the day after you swap them. The hen will keep laying every month or so and eventually her health will deteriorate and increase her risk for egg binding (death). Two hens together they will be fine. Majority of male pigeons will be more aggressive. Pigeons will grow, so you will have to adult birds no matter what sex they are. Every bird has their character. Fantails are fine if you are not planning to let them fly around.