Adding onto above, if you do have a pre-established tank by moving several items into your new tank you can immediately add in shrimps and snails as they have barely any bioload. However if you do not have another tank wait for a full cycle. Then I would add in plants and either inverts or otos. I would no recommend both as here will not be enough algae initially.
In a cycled tank you can add as many shrimps and snails as you want. Remember the food you feed them with contributes more bioload than the inverts themselves. However a general rule is 10 shrimp per gallon but many people house more with regular water changes.