Hi,
The general rule of thumb is 1" of fish per gallon of water. Remember to account for full grown not what they are now unless you will replace yearly. The stream does add to the overall total but I would stick to the minimum. Goldfish are quite messy so stay on the lower side especially until the biological gets going. 6 would be a good number to start. Do not assume losses, if you have a good set up there is no reason to think any will die. Feeders are small not sick, but do watch for deformities. With a small pond I would not suggest Koi they will outgrow the space. A full grown Koi can be 4', they do not stop growing because the space is small. Also keep in mind a plan for overwintering, a last minute rush every fall is going to become a pain eventually. Last thing to remember is that everything you add to the pond cuts down on gallons. The overall size may be 125 gallons but adding gravel, rocks, pots, etc. reduces your total so take that into account. Sounds like you are having great fun creating your pond, keep on enjoying yourself. Hope this helps

I made the mistake of getting myself 16 itty bitty feeder goldfish to check out the pond I started last spring, cost me under $5 so I didn't care if any didn't make it. In fact one was so troubled in the bag in line at the store it didn't make the night. I got so many cause they were so small and my pond so big I wanted a chance of actually seeing some fish when I fed them.

Today 11 of them are still thriving, the smallest is 4 inches long, the bigger ones are over 6 inches. They are wonderful fish but I would much prefer to have Koi instead.

I did get 2 little Koi for $9 that are now over 7 inches each, so my pond is really maxed out.

My real trouble starts next fall, my pond is too small to overwinter them there so I have an indoor tank, and THAT will be tiny after they spend the summer munching my water lettuce all day long.