Paul,
Please get back to us and tell us if what you have are actually bala sharks (google it, and see if the pics add up) - IF they are, then you may want to consider finding a bigger home for these babies. They can get up to 13 inches long, and also prefer to swim in larger groups, (of like 4 or 5), so these specifications are not met either.
Also, if by saying Pleco, you mean common pleco, this poses an even larger problem. These get up to 2 feet!!! 130 gallon tank would be preferable for these fishes.
Sorry if it stings a little, But it's really for the best.
When you do clarify and decide what you want to do with these current fishes you have, we will be more than happy to give you other suggestion for reasonable fishes for a 40 gallon tank
