Yes, you need to cover your HOB with a sponge, as the babies will get sucked in. If it's just a shrimp tank, you could use sponge filters instead.
You need about 6 or 10 minimum to get the population going. It would be a question of how many you can afford, and what deals you get on how many you want to start with.
Mine seem to like a planted tank. If you have some Java moss or trimmings hanging around, throw them in the tank, too. I think it might be important for the babies.