'Biotope' is the terminology for a tank that is a very close simulating the exact natural habitat of a river/stream/reservoir etc, so technically a tank that's simply full of South Americans (similar to mine) isn't a biotope, especially with the BGK in there.
Any Corydoras will be fine, don't think you need to stick to C. sterbai. Peppered Cory's are very cheap over here and a nice looking species in my opinion. You'll probably want them reasonably large though depending on the size of your BGK (which I presume is the big 40cm+ boy that you've got).