A week is not a bad yardstick, for a hobby [freshwater] in which many hobbyists don't have a full range of test kits. And a week is a pretty good guess, because it takes that long for the sandbed to adjust to the load.
However, in salt, having a full range of tests is a must.
You will want the following: a ph meter; a test for alkalinity, for nitrate, nitrite, and ammonia. In my own opinion, don't accept ANY nitrate or ammonia in the results. They're not good for the fish, even if they can survive them. Keep your alk at about 8.3 and the ph will ride about that level, too.
So yes, wait a few days, then test; and once you've tested clear, you can add another fish---and kudos for asking! Too many new salt tank owners just rush down and get half a dozen at once.
If you're keeping clowns, observe them carefully for the first few days, and pick out your fish carefully. They're susceptible to two problems, your old friend ich, the marine variety, and brooklynella, which is subtler and nastier, mostly involving sluggishness and a white film which some mistake for the clown's natural slime coat. Do not accept a lethargic fish, and particularly watch the gill action, to be sure it is easy. Do not buy from a tank with sick fish, period. Hope you have a long-lived and happy tank!