I don't think your dad is an authority on fish compatibility and suitability for certain tank sizes by any means. Cories can be kept with many different types of fish because they are peaceful bottom-feeders. However, red-tailed sharks get quite large (6") and are very aggressive. Minimum tank size would be 55 gallons.
You need to take the time to research each species of fish before you get it. A few minutes of googling will save you a lot of time, money, and hardship. Using a general calculator such as this one: http://aqadvisor.com/
can help you make your final stocking choices so that you don't overload the tank.
You should also research and understand a process called the nitrogen cycle. This is vital to having a successful aquarium. Here is our forum's article on the nitrogen cycle: http://www.bettafish.com/showthread.php?t=47838