Pairs of clowns can do well in many sitatuations, so long as a few criteria are met:
High water quality
Attention to maintanance
And if they're pretty much the only pair in the tank.
To give you an idea, i keep a happy, happy pair of true perc's in my 12 gallon nano.
It seems to me that you're new to SW, so i would recomend a 12 or 24 gallon biocube. They incorporate alot of things, don't need skimming, great shape. As close to 'plug and go' as you can get in SW.
If you have any questions, hit me up. I may be able to help.