Here are some questions and positive/negative answers that some have already mentioned that will help everyone help you (choose which letter applies to you) :
1. Has your tank gone through the nitrogen cycle (another way to say it: How new is your tank?)?
A. If it hasn't, then your fish will not survive for long, and they won't eat.
B. If it has, then your fish should be fine temporarily in a 20-gallon, and they're just being picky with food.
2. How long have your fish been in the tank? Did you get them and put them in the day of getting the new tank, or did you wait and put them in after cycling?
A. If you put your fish in the day of, your fish will not survive for long, and they are not happy right now, hence why they aren't eating. (related to the first question)
B. If your fish were put in after the nitrogen cycle and water quality, temperature, etc., is fine, then your fish are most likely just being picky. Try Deadeye's@ suggestion.
3. Are there proper hiding places for your fish?
A. If not: Mbunas live in a naturally rocky environment that has many hiding places; if you don't replicate that environment, then they won't be happy. Peacocks and Haps like open swimming spaces with minimal hiding spots, so a tank with sparse decor is fine for them. But for Mbunas like Yellow Labs, they need extensive rock work or caves in their tank to hide in. If they're stressed because there are no hiding places, they probably won't eat. Also, Texas holey rock would be ideal for your yellow labs.
B. If yes: Once again, they may just be picky, or it is one of the reasons above.
I won't state this one as a question, but you may have too small of a tank for 5 mbunas depending on their size, even if it's temporary. They may feel stressed and cramped, hence why they are not eating.
I wouldn't reconsider keeping Mbunas unless you don't plan on getting a much larger tank, like a 55-gallon or above. They get around 4-6 inches, and they need to be kept in a school, so they need a long-term minimum of 40 gallons, but preferably much larger. Since you mentioned that you are getting a new larger tank for your birthday, I don't believe this will be a problem. However, if you don't plan on getting something around 40 gallons or larger, then I would highly recommend taking those yellow labs back and figuring out something that is appropriate for your new tank. Remember to understock rather than overstock, especially with a new tank.
Others can give better info and ask better questions. I highly recommend taking the advice of most of these members as they are very knowledgeable and can really help you turn fishkeeping into a very enjoyable experience that will make you want to keep doing it for the rest of your life.
Good luck!