Nerite and margarita look very similar and are often sold interchangabley. >>Google image
<< search's can often be a reefers best friend.
, I would skip the bumblebee snails and stick with the nassarius as well as what has been mentioned. Both nassarius and bumblebee are primarily carnivor/detritus and will compete quite a bit in a smaller tank. If push came to shove, the bumblebee's are not above killing other snails to feed.
Just be sure before they are added the tank has finished it's cycle (both NH3
=0), do a large water change and be sure to drip acclimate the snails when you add them. As far as the cycle taking so long, leaving the shrimp will have added a great amount of nutrient in the system which is not bad, it just means that more bacteria is needed to deal with the load and may take a bit longer. Should be done in no time. 8)