I want to drop in a clownfish when my tank is ready, but not until then. Then add a clean up crew once my tank balances again.
Once your tank cycles, adding a single clown shouldn't throw anything out of balance. You shouldn't see a cycle at all - assuming the tank is cycled. If you add more than one fish at a time then you *may* see a little blip of ammonia, but that's it.
Guess I'm just saying I wouldn't hold off too long for a cleanup crew. They were the absolute first thing into my tank. For a 20g tank, I'd probably put 10-15 nassarius snails in for the sandbed, then put in maybe 4-6 astreas and 6-8 trochus. You don't have much rockwork to keep clean, and the tank is not heavily stocked, so I'd stay on the low side for cleanup crew quantity. You can always add more later.