Hi Drew,
My advice would be to skip the clown and just keep the two fish in the 10 gal. A 10 gal. with an established sandbed and LR can support the extra bioload but with a new tank, higher Nitrates from the additional fish waste and excess food can be a problem both for the fish and water quality for your corals.
Another issue would be compatibility with another Damsel (Clownfishes are Damsels) in a small tank. All 3 of these fish can be quite territorial and you might have some open warfare in there!
I would wait until Dad gets his big tank (it's inevitable) for more fish!
Dick
