I dunno, it might be ok for now, depends on the rock. The issue with api ammonia tests is that they tend to give false positives. So sometimes it says there is .25 ppm when it's actually zero. There is also an ammonia badge that you can buy which will change color if any is present so you don't have to keep testing all the time. I would pick up some ammonia buffer asap, I use one called Seachem Prime but there are other brands too. As your new fish poops and pees and breathes on the water he may add ammonia faster than the bacteria that survived the trip home from the lfs can eat that ammo up and make nitrates that don't hurt fish (aka the nitrogen cycle). The buffer changes the ammonia and nitrites into something the bacteria can still eat but won't burn the fish's gills hurt in the meantime.
Going forward, this is a good website to learn about fish before you buy them:
http://www.liveaquaria.com you can poke around to see what fish you like and there are filters to show you only "easy care" or "reef safe" fish. There's also a section for "nano" tanks, which is what you have. I'm thinking the clown will out grow a 10g really fast. Usually it's better to buy for the tank you have not the one you want, and a lot of stores are either dumb or greedy so you want to check the web before you buy. Also, once you see how few fish can really thrive in a ten, you'll be motivated to get that bigger tank running
A good option for a little tank is using distilled water from the supermarket at less than a buck a gallon. It's very pure and many nano reefers use it for their changes. I'd go ahead and get started if I were you.
Welcome to the hobby! We all started somewhere, you'll be a pro in no time