There was some method to my madness ....
The catfish was selected because it a) naturally exists in the same waters as goldfish (carp); b) I need something to clean the bottom; and c) the other "catfish" types in the same tank were all -according to the labels- considered 'jumpers,' needing a closed tank.
As for the Betta ... I have one book, devoted to Bettas alone, that asserts that bettas prefer cooler water, as in under 70 degrees. I have seen numerous Bettas exist happily in table top bowls .... whose water, when measured, almost always is below 70 degrees.
The Betta is, right now, the smallest fish in the pond ... maybe 80% the size of the goldfish, and a third the size of the catfish. The catfish looks about twice the size of the goldfish. While the betta has preferred to hide, in encounters with the catfish, the catfish has been in a positive rush to get away.
As with the catfish, I have been cautioned that the goldfish will become huge. Perhaps. Yet, since this pond is readily accessible to my cats - indeed, the cats were part of the reason behind this project - I do not expect the fish to get to be TOO big.
Right now, the cats are content to just watch.
With the increase in temperature, and light, I am currently dealing with a brown algae bloom. Filter change, water change, algaecide .... with another water change planned for tomorrow. Changes are in the 50-60% range. I also need to replace my water lettuce, which had it's roots stripped by the (conventional) filter used before.