Tropical fish often do great in ponds as long as the water isn't cold, you can keep blue acei,platies,goldfish and tigerbarbs in the same pond and the fish will look simply amazing when they come out at the end of the season!
Many of the aggression and territory problems disapear in ponds, which makes keeping them together easier.
guppies can eat algae from the sides of goldfish ponds,and produce fast enough that if there's a few places to hide,than you'll likely have more at the end than you put in.
Green terrors grown in ponds look way better than those grown in tanks,and green terrors do well with goldfish and koi.
Tiger barbs and tinfoil barbs tend to be better behaved in ponds.
Clown loaches grow faster in ponds, likely due to unprocessed algae in the ponds.