I really only had success with a bigger number of shrimp to start with. 15-20 would be the minimum I would want to start with. I tried with 8 and added a couple more to replace those I lost. Other than that Sponges over filter intakes so you don't lose babies, same goes for not having predator fish in there (once you have an established you can add some smaller predators if you want and the colony should be fine) and lots of finer leaved plants for micro fauna to grow in. Doesn't have to be moss, could be myrio, floating plant roots, hornwort, elodea, etc. just something with lots of fine leaves. Stable water parameters (doesn't have to be anything special as long as it's consistent).
If you want 100+ colony I would use the 10 gallon. I have a 5 and I would say there's probably 75 or so shrimp in there and it's a well established 2 year old and has spawned colonies in a 3g and a 25g and they have ~150-200 and ~25-35 shrimp in them respectively.