We have a Topeak MTX A-type on our road tandem (we had to get the A-type to go up over the rear wheel, hence we had to get an MTX-series rack). I cannot fathom that the clamp wouldn't hold. I'm a little concerned that you don't have enough seat post height, so you may want to shop in person. You could do an E-type (straight back) and possibly a V-type (slight drop), both of which are available in RX (road) forms. We use an RX-series EXP (I think, it appears to have been discontinued) bag, and it should be fine for a 50D with 18-135 (and a few primes if you were so inclined).