I doubt koi would "thrive" in a trout pond... However, koi/carp are hardy little buggers and I think the koi would manage maintain. Their growth rates would be greatly reduced though and perhaps some increased risk of disease. The one trout stream I fish supports both wild naturally reproducing populations of brown and rainbow trout. I regularly see and occasionally catch carp in this stream. A stream that stays cool enough to support trout year round but also supports carp and some darn big ones at that...
