Like Ruben said it might eat your adf's, but what was witnessed in the lfs was probably encouraged by hunger. A 4 inch tinfoil barb is close to the size where it might pick at an adf if it is really hungry. If you are sure that it is well fed, I would try it but watch for the first thirty minutes. I've had 12 inch tinfoil barbs in tanks with 1 inch tetras without problems but I've also seen hungry 2 inch tinfoil barbs tear into tetras at the lfs.