There's a reason they're both NU, you know. Ferrothorn is very common (for good reason) and Ferrothorns tend to carry Power Whip, which means a OHKO on both Gastrodon and Quagsire. If their threats are out of the way, I'm sure they can do very well. However, I'm pretty sure there are pokemon that do their jobs better. I'm not sure about Quagsire, but Storm Drain Gastrodon is probably outclassed by Jellicent, which has superior defensive stats and doubles as a spinblocker.