The platies and the filter media all at once will give you the best chance with a fish-in. The platies are pretty tough fish and will produce some ammonia to keep the media sample alive. It is a balancing act. You don't want so many fish that keeping up with ammonia is a terrible chore but you need enough to keep that filter sample alive and growing. The filter media without an ammonia source is just a way to kill bacteria. the fish without the bacteria is a way to end up fighting ammonia daily. You would be best to go somewhere in between.
Ok. So we went with the fish in cycle. We put a serpae tetra and gold barb in 5 days after setting up the tank. So far no problems. Fish seem to be doing fine and are eating regularly. I did a 25% water change 1 week after setting it up. I plan on doing 5% changes every other day(until cycle is finished) and will continue to do the 25% change weekly. We are going to add 2 more fish 2 weeks from last thursday. I have read that serpae tetras and gold barbs should be kept in groups of 5 or more. Do I need to stick to that number or is a smaller number such as 3 or 4 ok? Also, how do I know when the cycle is finished? Hope all stays well and thanks for the help.