1st time posting; compatibility questions

[Cecibella]

Hello. I've been registered with the site for a couple years, and often peruse the contents, but have never posted in the forums, until now.
I love catfish, 2nd only to cichlids, and have several: 2 Synodontus cf_polli in my Malawi cichlid tank; 3 Pimelodas pictus and 1 Horabagrus brachysome in my New World cichlid tank; several varieties of Corydoras in a community tank; and 3 Ictaluras punctatus in my 6,500g koi pond.
Which brings me to my questions:
1: Can other breeds of Synodontus co-exist with my S. polli in the Malawi tank? I'm especially interested in upside down catfish, and featherfins.
2: does anyone know of any interested, smaller growing Asian catfish that would do well in any of my tanks, or any interesting catfish from other continents that would do well in my tanks? My water is fairly hard, with a normal PH of 8.4.
3: Are there any larger-growing and interesting catfish that would do well in my koi pond, and survive Midwest U.S. winters? The pond is about 42" deep, but of course freezes over during winter. We keep an area open to allow for gasses to escape by using a floating stock tank heater, but the water is still very cold.
Any input of advice would be greatly appreciated. Thanks

[Cecibella]

I should also say that the New World tank is 150g long, the Malawi tank and community tanks are both 90g. Thanks

[Richard B]

Any of the lake species of synos would work in a Malawi tank but i'd avoid riverine species - particularly nigriventris.