If you've never had a tank bigger than a 30 gallon, my suggestion to make water changes easier is to get yourself a python. Makes life super easy. I do all of my water changes (400 gallons worth) in about 2 hours using one of these.
Thanks to everyone for the replies. I wonder if bala sharks will eventually eat my mollies. One thing I enjoy about mollies is breeding for a specific look.
You're going to need a much larger tank if you plan on keeping balas, but yes, I do believe they would most likely eat the mollies eventually. They are omnivores and will eat just about anything that will fit in their mouths, which for an adult specimen (~12-14"), would probably include small mollies.