The "pink" gourami's you are speaking of sound like pink kissing gourami's aka, kissing fish. They get this name because of the movement of the mouth when they spar with each other or during spawning.
These CAN be kept with the oscars so long as they are large enough that the oscars can't/won't eat them. Kissing gourami's get quite large, about 6 inches in diameter, can be somewhat aggressive if feeling crowded, or if smaller, weaker, more peaceful fish are in the tank.
I will agree with Blue that 30 gallons is not suitable for 2 kissing gouramis and 2 oscars, of any size. Watch your water quality carefully, it will likely spike all at once, and ALL of the fish in that tank are then at risk.
Kissing gouramis are sensitive to water quality, as are oscars.
For diet, I would work with tropical flake food, brine shrimp, bloodworms, and/or frozen formula 1 food. The formula, brine, and worms are all suitable foods for the oscars, also, the formula food being the more nutritious food for them even over pellets. With the garlic, it helps to boost the immune system, which helps to protect them from common illnesses and disease issues. Be VERY careful not to overfeed them, especially in a tank of that size. With the frozen formula 1, put a few cubes into a cup of tank water and let it thaw completely through before feeding it. (this usually takes 15 - 20 minutes for a few cubes) When puting it into the tank, break it apart into small "chunks", your fish should love it and have no problem eating it all very quickly. You'll want to make sure all of the fish are getting the food, as both kissing gouramis and oscars can be quite stingy, greedy, and aggressive during feedings.
I hope this helps...
Here is a link to a picture of the pink kissing gourami http://users.tkk.fi/~mputkone/aquarium/Gour01.jpg
let me know if this ISN'T the fish you're speaking of, and I will do some research.
Blue, I'm not familiar with gouramis being dyed... there are certain traits a fish needs in order for dying to "work" the way it's intended. Even if it were dyed, it would not make the entire body one solid color. The dye is something that runs through blood vesssels and organs, which is why you see it more as "stripes" or "blotches" in dyed fish. Tetras, danios, and glass fish are the most popular for the dying process, which is very cruel and painful for the fish, and the survival rate of the injected fish is very low. The dyes also tend to fade out as the fish grows, so it's only temporary color.