If you get a juvie oscar for that tank I would have my doubts as if it would even become large enough in size to eat them. Oscar's, despite the large size, are rather mellow and I would think would get bullied quite a bit by the more aggresive mbuna. If it did make it to a pretty good size then I would say it would eventualy eat it's tankmakes. Again not because of aggresiveness, but they will often just eat any fish that is small enough fit into it's mouth (and some that aren't).
I would say add more mbuna to the tank. Many people use controlled overstocking on mbuna setups. It's not as easy as just tossing a bunch more fish into the tank, but pretty close once you understand what they need.
I had a rainbow shark for about 4 years, and it took it nearly that long to reach about 4 inches.