Hello,

I am in the process of installing a 100 gallon pond in my yard.  I am looking to understand what kind of fish I can have in the pond.  Can you help?

I don't think koi are such a good idea.  100G is very small for a pond.  Can you answer these two questions:
1. where do you live?
2. are you installing a filter and heater?

living in Texas, the pond is probably not prone to freezing, so the that shouldn't be a problem.

If you want to keep anything live in the pond, you're going to need a filter of some kind.  A skimmer box with a pump into a filter tank, and on to a water fall would be perfect.

Anyway, in a 100G outdoor pond, assuming you do add a filter, you could keep 4 or 5 comet, subunkin, or sarasa goldfish.  No fancy goldfish or koi.  You could also keep rosy red minnows or white cloud minnows.

One last thing to consider - if the pond is exposed to the sun, the small amount of water you have is going to get very hot very fast.  I would recommend putting some kind of covering over - like a pergola, or similar, AND add some floating plants to let the fish hide from birds and other animals.