I was told that fish dont outgrow their space.
The old myth persists, sadly. That would be like saying "get a baby elephant - it won't outgrow your house!"
I suppose it is possible to keep a fish in poor conditions, underfeed it, keep it malnourished and it won't reach it's full size potential. But that's hardly what any of us would want. And people who sell fish for a living should have the best interest of the animals at heart. Most people would never buy koi if they knew they understood the potential for growth. Goldfish and shubunkins make great pond fish - very pretty, friendly, and lively and they add much less stress to a pond.
Completely understand about the flooding situation - you're being a responsible pond owner, which we can all respect!