golden chinese wels

I’m about to purchase a golden “wels” catfish but the current owner is unsure on it’s particular species at first I thought it was s. meridionalis but now I’m unsure does anyone have any ways of correctly identifying them so I know whether I’m going to end up with a 3ft or a 10ft fish eventually

I really want a golden chinese wels for my 21,000 gallon koi pond (32.5 ft by 14.5 ft by 6 ft), but I am worried about the wels eating some of my koi. I just started the pond, and I only have 2 large koi in at the moment. 1 17 inch male goshiki and 1 20 inch female showa. Would a larger...