Glad the explanation helped. Since it didn't confuse you, I'll add just a little more! Because those "A" and "B" bacteria are constantly looking for food to eat (ammonia and nitrites), this is why it's important to keep "feeding" the tank after the cycle is complete. If you finish your cycle, and wait a month to put fish in, the bacteria populations aren't going to have anything to eat during that time, and will most likely die. So when you put your fish in a month later... BAM... you start another cycle. So moral is... if you plan on keeping a tank with no fish for a while after the tank cycles, don't forget to put a pinch of fish food in there every few days to give the bacteria something to eat!
Regarding your CUC, for a 29g with 30lb of LR
, I wouldn't think you'd need more than 6-8 astrea or trochus snails for the rock/glass and maybe 10-20 of the little nassarius snails for the sand bed. I'm not really good at guesstimating cleanup crews, so maybe someone else will jump in with some ideas. I wouldn't overdo it though... you can always add more later. Regarding crabs... one or two would probably be just fine. Once you start adding corals (if you're going reef), the clumsy hermits dragging their shells around, or falling off rocks, can cause havoc with the corals. Or at least they did in my tank.