Autumnsky is right, you're inevitably going to have baby guppies in your tank. You don't *have* to have a second tank for them, but some may get eaten if they're raised with the adults. To give them a better chance, buy some java moss to give them hiding places until they're too big to be snacks.
Also I'd recommend trying to get two females for every male, otherwise the males might pester the ladies to exhaustion.