Emperor's are pretty big tetras, getting to 2 inches at maturity. The ancistrus gets to 4 to 5 inches. It would have to be an either/or thing between the catfish and the emperors. Personally, I like a completely different idea for your 20G, though it might not sound appealing initially to you. I think you should keep looking at tetras, searching for a still smaller tetra that you still like but that will look good with the neons/cards. I would try to do low-tech planting and get the tank pretty crowded with slow-growth plants. Two larger shoals of smaller tetras moving among these plants would produce a very nice picture. If you could find a tetra with similar size to the cardinal, you could stock each shoal to about 10 fish I think. If the chosen fish grows a bit larger then the number of that species might need to be left at 6 or so. Just a thought.
~~waterdrop~~