A good substitute for the conchs would be nassarious snails to keep the sandbed tidy, they are excellent sand cleaners/stirrers that are usually buried in the sand until they smell food then here they come.
The best way to determine if you can have more inverts would be by how much food is available for them, if waht you have is keeping the tank nice and tidy then adding more would cause a food shortage. The key is to find out just how many you need without overdoing it. BTW, bumblebee snails aren't good cleaners, they don't eat algae but are carnivores. I like margarita, astrea, trochus, and cerith snails for cleaners. Turbos are excellent cleaners but they tend to knock things over all of the time in a nano kinda like a bulldozer. HTH, Skip