The tank is not larger enough for a Bala Shark. Bala Sharks like to school as well, so it is really a bad idea to get just one and then keep it in a small tank. A Common Pleco would quickly outgrow that tank as well. You would need to get a Dwarf Pleco like a Bristlnose or a Rubberlip Pleco that stop growing at 4 inches.
Your tank is only cycled for three fish worth of waste. Adding more fish will will likely cause some ammonia/nitrites as additional bacteria grows to deal with the extra waste load of more fish. When you get more fish, I would get just two, then wait before buying any more.