Depending on the brand, ready made beer can be pretty high on hops, inhibiting some desirable microorganism growth, and pretty low on easily accessible sugars. I think it can still work, but the success rate will probably be low.
Here, I provide a medium rich in sugars, low on hops, with some acetic acid and alcohol - all selecting against the majority of stuff that I don’t want.

I’m not the expert here, but from what I gather the yeast flakes provide amino acids and such. Yeast nutrient would have been better but that I also did not have on hand. And yes, it’s Hefeflocken, exactly.

The oil on top is there too inhibit mold. Yes, for multiplying, yeast needs oxygen, but there will be some dissolved already I guess. After all though, this is more of a selection than multiplication step.