Well first of all, if you have fish in your pond, don't put in ammonia. The suggestion of ammonia assumes that you are starting a new pond, not just a new filter.
The best bacteria for your pond are the ones that are already living in your environment. Nitrifying bacteria are in any healthy soil and in any outdoor body of water that has been sitting around for a while, from a bird bath to an ocean. The few species of nitrifying bacteria that you can purchase are the ones that are easiest to grow in culture. They are great for aquariums, but totally unnecessary in an outdoor pond and not usually well adapted to your climate. Unless there is no way that dirt can get in your pond and it is completely secured from wildlife, your pond will be naturally inoculated. If you already have fish in your pond, the bacteria are there. They will populate your filter without any action on your part.
If you have a new pond, you can put in ammonia to feed the nitrifying bacteria that are there or will be there soon. They will multiply quickly with all the food. Test your pond regularly for ammonia and nitrite, and when both are gone you are ready for fish. That's called fishless cycling.
Alternatively, you could just put in a couple of little fish. By the time they have put out enough ammonia to be a problem to them, there will be enough nitrifying bacteria to handle it. As you gradually add more fish, your bacterial population will increase to accommodate them. This method works fine but it's slower.
Need to add start-up bacteria to my media. What do you guys use? What's the best? 
