I would keep the 20 as a QT, you might keep
soft corals in it with no problems BUT responsible reefing means doing everything you can to insure the health of the animals you adopt and that pretty much means NO COPPER with any inverts.
Not to rub salt into the wounds further but a 20g tank is way too small for a pair of Maroon clowns, they can get 6" easily and are among the most territorial and aggresive of the clownfish. The females can be the epitome of B*****ness and in less than a 55 with plenty of rockwork, the smaller male can be in serious jeapordy. Getting them small and raising them together is no guearentee, one of the pair will turn female and theres no reasoning with hormones after that, sorry