In my opinion and experience, you really cannot or should not cycle a tank with a shrimp or a snail. Glass shrimp are actually a marine shrimp and ghost shrimp are a freshwater shrimp. They are small and do not make much waste at all. Snails would be the same and are more 'fragile' as far as water quality is concerned. There are a lot of good methods to cycle a tank without putting any living creature in jeopardy which you can read about here:
http://www.aquariacentral.com/forums/showthread.php?t=84598
There is no way to put a time line on a cycle and so do not fall into a 4 week deadline for your tank. There are so many variables within the process so the best and truly ONLY way to know when your tank is cycled is by using a test kit to monitor ammonia, nitrite, and nitrate levels. When ammonia and nitrite are at 0, and nitrates are present in the tank, it is cycled. You can only remove nitrates by using some nitrate removing filters or by water changes, which is the fastest and cheapest method. While ammonia and nitrites are toxic in relatively low levels to your fish, nitrates, while still toxic can be present in higher quantities in your tank and so a weekly water change is USUALLY the case. Things like overstocking a tank or over feeding are going to raise nitrates quickly and so in those cases you may need to do more frequent water changes or fix the problem.
Rainbow sharks, in my experience, are labeled semi-aggressive, but this is really only because they do not get along with other rainbow sharks and/or other fish that look similar to them such as the red tailed black shark. You are right, they are not sharks at all so you can mix some community fish in the tank with them. While they are not really considered 'bottom feeders' they do spend most of their time in the lower portions of the tank. For this reason, it is important to not overstock the bottom area of your tank with lots and lots of bottom feeders, or the rainbow shark may feel the need to defend some territory. Otherwise, they will get along with most anything. I have a larger one in a 55 gallon cichlid tank. I also do not agree that they grow super fast and would believe that you can keep one in a 20 gallon tank for quite some time. I have had mine since January of this year and it is only 7 or 8 inches long and able to turn around in any 20 gallon tank. This is all especially true if you are going to keep it in a tank by itself.
TropicalNorth,
While I understand that you have good intentions, I do not think that your advice to cuticom is very complete or really very accurate. You may need to spend some more time reading about how to cycle a tank and just how important water test kits are to the hobby. It is also not advisable in the least, from my experience and in my opinion, to cycle any tank with an invert of any kind since they are sensitive to water quality and snails can be burned much more than a fish by ammonia. Ghost shrimp are not the longest lived creatures in the world either, but this doesnt mean that they are disposable and should be put into a tank which will not have a healthy environment. They never stop and eat constantly so putting them in a cycling tank would be hard because they would need a lot of food, and in which case, one could just cycle the tank using fish flakes anyways.
