If the tank is properly cycled there's no reason you cant add them now

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The reason people wait a few months before adding shrimp is just in case anything goes wrong and you get a 'mini cycle' (like say the filter breaks down), because shrimp are pretty sensitive to ammonia and nitrite spikes.
A 'mature' tank basically has more bacteria so can deal with small problems like that easier.
So if you feel confident, add them now, if not for any reason, wait 3-6months

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My advice on shrimp is just to make sure you know which species you are buying. Amano shrimp and cherry shrimp are good, pretty hardy choices for a community tank (providing you only keep small fish), if you live in the US ghost shrimp are quite common and very, very cheap.
Another thing is that they need hiding lots of places away from fish when they shed their skin, so lots of plants, bits of wood, coconut caves and the like area good idea

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