I personally love Stress Zyme. I use it after pretty much every water change. It is beneficial bacteria in a bottle. Now i have just recently heard, and naturally cant remember the names, that there are basically two types of bacteria that break down ammonia and nitrate. One, the type Stress Zyme contains, does about 10% of the work. The other, which i cannot remember the name, they have just released as a new product that does the other 90%. So i would completely encourage you using Stress Zyme as it does make a difference, but at the same time dont rely too much on it. Still wait atleast a week or if you insist upon adding fish immediately, do it very sparingly. The type of fish you want makes a big difference too as some are more hardy than others.
Also, if you have ever heard of a product called Cycle, it is basically the same exact thing as Stress Zyme.