If you like Bob Fenner's take on things, he recommends the Sohal:
http://www.wetwebmedia.com/Asohal.htm
Not so much the Clown:
"An aggressive species that causes trouble often with growth in aquariums."
From
http://www.wetwebmedia.com/badacanthurusaq.htm
If you value the peaceful environment your Naso and Regal share you may want to consider how that is likely to change, at least in the short term.