It doesn't really matter about your flow.. Generally it is a good idea to have the filter disturbing the surface it aids in the gas exchange (like an airstone) but not a good idea to rely entirely on it (ie if your filter fails and the surface isn't getting disturbed all your fish die). This wont be an issue in a tank your size with your stocking but can become an issue in hotter weather in heavily stocked tanks.
Shellies seem to handle a bit of flow I am not sure what they like in nature I am sure you can find some articles on it somewhere, or maybe videos of them in the wild to compare it to but they seem very tolerant, same with the jullies.