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I'll make this short to not bore people with background. I'm freeing up space in my 75 long (48" x 18" x 20") and want to make it a mainly catfish tank. The base stock will be 6 giant danios, 1 striped raphael catfish, and a gibbiceps pleco I'm growing out since they're already in there and I want to keep them. The giant danios are about 2-2.5 inches, the raph is about 3.75-4", and the gibby is about 4".

Some of the fish I was considering (That I have readily available) are a 5.5" Bagrichthys macracanthus, a 3-4" parauchenoglanis ngamensis, a 2.5-3" Ameiurus nebulous (I'd be growing the bullhead out obviously they get to large long term), and a 2-2.5" synodontis Angelicus. I know I can't get all of them but I'm looking for a good combo of them and the ones I already have. I'm leaning towards getting the lancer and the bullhead plus the ones I already have but that may be overstocked.

Here's my tank specs:

Volume: 75 US gallons
Dimensions: 48" long by 18" wide by 20" tall
Substrate: loose gravel
Lighting: 3:00 PM - 9:00 PM plus ambient daylight
Filtration: Fluval 406 canister, I'm willing to get more if necessary but nothing to pricey.
WC schedule: 40-50% weekly
Temperature: 75-76 F
Ph: 7.6

Since you plan to keep a variety of species from all over the world your possible choices are almost endless.
I personally wouldn't stock it with the Ameiurus nebulous(Bullhead Cat). Mainly because it is a coldwater species which has a rapacious appetite, a mouth to match and it's ultimately large size.
If you just stuck with the species you listed your tank would be heavily stocked once the larger species have matured.

The gibbiceps pleco is just something I'm growling out until it hits about 10" then it'll go into a 180.

For the stock I wound up doing 6 Giant Danios, 1 Striped Raph, 1 Gibbiceps, and 1 Yellow Bullhead.