housing breeds of freshwater fish

Question
QUESTION: I recently bought a very young iridescent shark and an oscar, both from the same tank at a local Wal Mart. They sales lady said they should be fine together and sold them to me in the same bag. Now I am not so sure this is a good idea. Some people have told me that the oscar will grow faster than the shark and eat the shark when it is capable of doing so. Other people say that is not so and as long as they where together all their lives they should be fine. I would like some information about this situation, as i love both fish and wish to see them live happily together. Thanks
ANSWER: I don't know what size tank you have, but I can tell you an ID shark needs 200g or more.  A fully grown Oscar needs 60-75g minimum.  Pet Stores are always willing to sell you fish, regardless of your tank size.  I don't know if it is because they think you will buy a 200g tank from them when your fish gets to big, but it doesn't make it right.  I would return the ID shark and if your tank is smaller then 60g, I would return the Oscar too.

---------- FOLLOW-UP ----------

QUESTION: The size of the tank (money) is no option here, I love my shark and can get huge tanks relatively cheap anyways, I have a 100 gallon on the way and plan on purchasing a 200-250 gallon, or having a pond built in the living room within the month, whichever will happen quicker, i love animals and will do anything to make them happy. I just want to know if the oscar is going to grow faster than the shark and eat him up before the shark gets a chance to grow. If that is the case i will have a 100 gallon and a 200 or 250 gallon whereby I can seperate both fish until they can live together and not makes meals out of each other!! like a say, money and sizing of tanks is not an issue here, the sky is the limit, i make lots of $$!!

Answer
My Oscar grew from about 1" to 12" in less then 2 years.  I have never had an ID shark, but I think that because they live longer, they probably do grow slower.  I would keep them in separate tanks.  I am not sure if they will tolerate each other when they are older.  I had my Oscar with a parrot and a pleco and they were all fine, but it all depends on the Oscar and the only way to fine out is to try.  If you are planning on keeping other fish with either of them, make sure you never feed live food.  If your Oscar never gets it in his head that you can eat other fish, he will be less likely to try.