IMO you can only put one maybe two fish in the six gallon but two might be pushing it for such a small tank. Try maybe one small percula clown if your into those. As far as coral it all depends on the lighting you have in the tank. A quick description of what you have as far as lighting goes would help.
I think the rule of thumb for lighting is 4-7 watts per gallon. But mediahound is more experience with coral than I am so he might be able give you some more suggestions now that he's got an idea on how many watts you have in your setup.
you can have 1 false perc and a firefish in the tank..false perc look like regular perc's somewhat but dont get as big..also to keep aggression down for coral get a small nem or frogspawn..something the clown can host..
emerald crabs from your 50g? Well how many I only have one in a 12g nano, and in a 6g it might get aggressive b/c thats such a small tank, even though they are reef safe and calm, but thats a smaller tank. If you do be sure to acclimate them.
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