Add some hermits and feed them a few times per week, this will produce enough ammonia to keep the cycle going. A dedicated q-tank that has fish in it all the time, is not a q-tank. One of the benefits of a q-tank is to allow a fish to acclimate to captivity all by itself, or at the very least on a level playing field with another fish, having a fish in there all the time, your looking at aggression issues from the established fish and increased stress from the new fish.
If I were to choose something it would be a very peaceful non-predator, possibly a scooter blenny or a firefish. The two fish mentioned above are, not only unsuitable for a 20g tank, they are also unsuitable for the job you want them to do.