Yes. The smaller clownfish are males, and will naturally take that role if there is a bigger clown present. The size difference is to reduce competition for 'top spot', as the larger one can easily assert its dominance for the female position.
Yes, the smaller of the two will usually become a male, while the larger fish in the pair becomes a female.
The reason you've heard not to put them together at a similar size is because if the two fish are fighting for dominance and one isn't noticeably bigger than the other, it drags out the fighting and ends up causing more stress and injury to the fish. That said, it wouldn't be a great idea to put a full-grown female with a barely mature 1 1/2" fish, either, as it's possible the female will kill the smaller fish before they establish who's who. Clownfish can be extremely territorial, and depending on your female, you might have a LOT of trouble adding a second clown to her tank. Use caution, and have a hospital tank ready.