Hey buddy! I have to say it can be done but NOT suggested. You have to be careful what the LFS is telling you. They may be intentionally leading you wrong to get the sale or they may NOT know better. Keep in mind they aren't in the business to keep fish long-term and yes they probably have MANY clowns in a med-small tank but they want them to LEAVE Asap!!
I have a clown in my 12g NanoCube and have had it in here for (counting the months on calendar) 21 months this very month. I put it into the tank March of 2007 and he's still going strong. Is this the most AWESOME home for him? I don't know for sure but it's healthy and has so much personality that I find it hard to believe it's not healthy. Now to add some input.. I do WEEKLY water changes of 10% (or more) and he's the only fish. At one point there was a Citron goby but it went into the rock work and I never saw it again (clueless what happened). Then I had a Neon Goby in here for about 9 months and it just vanished (clueless).
So CAN you do this? Yes... IF you make some decisions...
A) it depends on the type of clown (some can get very large)
B) if you're willing to commit (it's a LONG term commitment) to hold up your end of the bargain and do what's right to maintain this system.
C) I think ONLY one clown for sure. With such a small system you need all the luck/help you can get and adding 2 clowns will over-load your system in short order and your tank will decline from that as your work load on the tank will increase..