i wouldnt use a filter. just an air stone for water movement. chang the water once a weel and u should be fine. the plants will ''eat'' amonia and nitrates.
somewhere around a 30g+ for those 10 bamboo shrimp(Atyopsis mollecinsis right?) they get big, around 4". otherwise for the 10 ghost and 10 cherries i'd say a 2.5g