I have an oceanic 50g which measures 30L x 21H x 18 deep. Not ideal I am sure. Can I however use this tank if I stock appropriately? Perhaps only five or six 3" - 5"
The length of the tank is recommended not in terms of being able to stock more, but being able to stock a wider variety of fish. The more aggressive mbuna are, the more length they need in the tank because they have that much more distance to run from each other and get out of site. Your tank has a nice footprint non-the-less, and you can work with this, especially if you take advantage of the extra depth (18" instead of the usual 12") when decorating the tank. Since the tank is short, however, you simply need to go with species that aren't too aggressive. Some ideas of this can be
Labidochromis Caeruleus (yellow lab, electric yellow), Iodotropheus Sprengerae, Pseudotropheus Acei, and Aulonocara (peacock) (most, but not all, types). You can fit about 8 fish comfortably - I'd do something like 4 Acei and 4 labs, or 4 labs, 3 rusties, and a single Aulonocara male. The choice is yours. You also have the option of some catfish - perhaps some synodontis petricola, as they are small and do great in these tanks. Catfish stay out of the way, and with 50 gallons of volume you can add a few of these on top of, not instead of, your mbuna.
Whatever you choose, don't randomly buy mbuna unless you are sure. In a tank that's too short aggressive fish become even more aggressive, and you'll just end up with problems. There are a few other species besides the ones I've mentioned that may work, but those are by far the best alternatives for you. As far as decorating the tank - the higher you stack the rocks, the more height they will use, but they tend not to rise much above the rock peak - unless they're getting chased there or under duress and trying to avoid an aggressor.
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