10 gallons are cheap, I'd buy a separate tank just in case you needed to medicate. My reading has told me that copper can get in the silicone of the tank and always be present in that tank. I don't think copper would be very healthy for fry.
But when it's in there you just watch it to make sure it's not sick, and how long depends on if it gets sick. I'd say 2-3 weeks if no signs of illness. If it's sick, then 2-3 weeks after the disease is cleared up. How many? You could get away with a handful of small fish at a time in a 10 gallon, but a full grown emperor angel would need a bigger QT.
As far as corals - I have no clue. I just drip acclimated mine for a few hours then in they went.