Goldfish and koi are really different creatures when it comes to how many you can have in a pond - the biomass of a koi (and the resulting waste they create) is incrementally larger than a goldfish. You're fine with that many Goldies in your pond !
I'm not sure what you plant is that's growing in the pond, but if it's been there this long you can assume it's fine for the fish. Just don't let it overtake the pond and you'll be fine!
Lilies like it warm and sunny for the most part - there are a few that will bloom in low sunlight, but for the most part they want sunshine. What kind of exposure do you have for your pond? And there are TONS of other pond plants you can try other than lilies if you find those don't do well in your pond.