Tragically it is what a lot of people go through, Pet stores are really only after your money. Sad but True, there are very few who actually care for the long term happiness of the fish once they get them in.
If you can get a 55g then you can still have a tank of Lake Malawi cichlids and I can and will help you along the way. The only thing the 29g is good for is the smaller South American shell dwelling cichlids, mbuna will not be happy in there as it is just too small for them.
The yellow labs would make a good starting point for a 55g as they get along with almost anything from Lake Malawi. You would want to stick with mbuna though as they are too active for the more peaceful peacocks. Even with the fish as small as they are now, it does not take long for them to grow and start spawning, once they reach spawning age, all hell will break out with the zebras if there are any females.
Truthfully, your fish store should take the fish back and offer you a store credit. You can also try posting in the classified section of the forum and also perhaps aquabid.com
as well. You would be better off keeping the yellow labs and getting rid of the zebra.
Long term the yellow labs will breed with the zebras and leave you with hybrid unpredictable fry.