silurus glanis

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 had a chance to get an albino wels catfish but I know nothing about this cat. It is still in shop, around 5". So many question running through my mind right now. Does it exactly the same as Wallago in terms of behave or feeding, etc? Can they be mix with other catfish, eg piraiba? There's a...