Fancies and commons generally get along fine. The problems come (as Lupin mentioned) during spawning, feeding and panic.
Any spawning, even if it's not between fancies and singletails, will cause the whole tank to get excited. Males of any type will chase the spawning female. If your fancies are among the male suitors chasing, they can get banged up and bounced around by the more powerful singletail males. If the female being chased is a fancy, they could kill her or seriously injure her. Adding soft live or silk plants to the tank will help to keep the fish safe, giving them something soft to spawn in. Add plants as soon as you see any chasing so the fish can quickly spawn and won't wear each other out.
Feeding is less of a problem, especially if you feed a sinking pellet. I feed different types of food - fresh veggies, pellets, live food, gel - to ensure that even if fancies are slower getting to the pellets, they can graze more on the veggies or get more live food.
Panic is the final issue. Fish can panic during netting, during a move, or even just when frightened by a loud noise, earthquake, etc. Commons, being so large and fast, will race around the tank like crazy and can injure the slower fancies. You can reduce the chance of injury in this situation by keeping sharp or rough objects out of the tank.