I have both aswell, but my biggest tanks 220, 75,56 are sw. Shrimp are more expensive then crabs but much safer with other live stock. For snails get a mix, cerith,narssisus, astrea, nerite, etc
People mix them all the time. They will kill snails yes, but its a war of attrition. In my big tank i have a few softies and snails so i keep shrimp instead of crabs. 2 pep and 2 skunks. In my 75 "aggressive " tank i keep hermits bc shrimp would be dinner lol
A variety of empty snail shells, yes. Alot of lfs will have empty shells in there tanks from dead snails before. Some will sell you those or just give um to u
Here I'll speak to it lol. There a good part of a cuc, however they will kill snails for shells. Some peeps have had them eat certain soft corals aswell
Quick answer, you can't really over filter. Under filter definitely, but unless your fish are getting blown around there is no over filtering. Now your fish list especially the trigger, hippo, and parrot thats going to get a bit excessive especially if your 120 is a 4ft tank.