How long have you had the mandarin? I'm pretty confident that 45G is too small to naturally grow enough copepods to feed an adult, so if you're in the first couple of months or the mandarin is still young, you may not know they'll dwindle quite yet.
A leopard wrasse is a larger animal still, which is probably just too large for your tank without feeding requirements, so I would cross it off the list until you upgrade.
Make sure you see your mandarin eating something else - if you're feeding a reasonable amount of things they could eat, they likely will start, and I think this will be important to the long term health of your existing mandarin. They can learn to accept frozen food or pellets, but my experience has been that the first one they learn to accept is frozen bloodworms - just make sure the pumps are off for a few minutes so they can scoot around and actually find them on the bottom.
If you can confirm your existing mandarin is eating prepared foods and you're feeding them every day, you could consider getting a second under the condition that it can also be trained onto prepared foods. If you have a male and a female they can certainly coexist (the big difference is the spike on the front of the dorsal fin), but they sometimes fight when first introduced, so after you have the new one eating prepared foods too, try introducing them in a clear acrylic box with vent holes, and once they leave each other alone, you can let the one in the box out. With my current pair, also in a 45G and adults, but fed twice daily with frozen foods they were trained over 3 weeks to accept, I added the male second, and the female acted quite aggressive towards him for a few hours when he was in the box until eventually being okay with him.
