Teresa
Koi need more room than goldfish. Also the water volume will indicate how many the pond will be able to support. One of your biggest issues with fish though is that the pond will need to have some type of filtration.
You don't have to have a waterfall. There is no wrong or right way, it is whatever works for you. We have a bubbler that we turn on when the water temps are over 80 degrees as the warmer water has less oxygen content than the colder water. You could just run a bubbler in a pond or perhaps a fountain. There are a number of different things you could do. I have looked at the spitters myself and would love to have one, but haven't found the perfect one yet.
There are a number of different types of plants and waterlilies that would do well in your pond too. I have a couple of favorite waterlilies myself and have found they trade as quickly and as easily as other plants.
