2wpg will do for your moss. Any higher and I think without sufficient plant mass you'll run into problems.
Shrimps can do without a filter, but I still reccomend you go for one since it provides water circulation, something that mosses do well with.
My vote? Go without CO2. Much less problems. Go with a small internal filter in this case (those Eden ones in blue packaging), and keep the water cool. Your moss should grow beautifully this way.
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