You can start with as many as you'd like. They will generally breed and populate the tanks as long as they are a type that breeds in fresh water. I would probably start with 10 or so in the 30g and upwards of 30 in the 100g.
Yes, they are scavengers and will eat anything they can get their hands on.
Larger fish as well as cichlids and loaches aren't really compatible with shrimp so that's something to keep in mind.
I know my praecox rainbows have driven my shrimp into hiding. The boesmani might eat them quite readily. If they aren't dwarf gouramis in your tank they will probably eat them as well. I would suggest going with ghost shrimp because they tend to get a little bigger.