Why not ask someone on the forum?
Jewels usually range from 4-6" (Depends on species), and are aggressive (Very very aggressive when breeding) . They are afircan river cichlids, so their water requirements should be similar to those of New world cichlids (Neutral-soft pH, around 78 degrees) A 30" is the bare minimum, because these fish need space for territory. If you have a large enough tank, they can be kept with medium sized new world cichlids, ours lives in a 29g high with a few rainbow cichlids, and he is doing pretty well
We feed our jewel cichlid sticks and shrimp pellets, he is a 6", 7 year old male
IMO, getting information from individuals is better than getting it from a book, because you can't ask books questions
Hope that helped