I'm not really a fan of cycling with fish, that kind of became outdated a long time ago IMO. But it's a pretty big tank and it's not likely that 2 clowns will have a problem, but please do keep an eye on them.
And yes I think your LFS took you for a ride

But nothing wrong with that, you probably planned on having some clowns anyway! Them telling you to cycle with a Yellow Tang is suspect though.