I'm no expert on this, but I know some soils produce a lot of ammonia in the first weeks (especially ADA). Maybe it's safer if you presoak it for a while in a bucket with daily waterchanges.
If you have a lot of well established plants and filter I suppose those would soak up most of the ammonia.
I have from time to time added in more aquasoil. I use Tropica Aquarium Soil (not the powder) and really like it. If I'm just topping up by a cup I don't bother with prewashing, just dip the cup of soil into the tank to get wetted in the cup and then slowly pour it down where needed without any other precautions. I've never noticed any adverse effects on livestock in my relatively heavily planted well-established setup. If you're nervous you could build it up slowly over a number of days rather than going in with a whole bag at one go if you're not in a big rush.
It’s a bit of a how long is a piece of string question. Adding “a lot” will undoubtably stress lifestock, the filter and plants with the high amount of leaching. However if it’s just “some” if you keep up extra water changes, then in my experience spikes can be minimised as the filter is precycled and the soil and plants are doing all the heavy lifting in the tank already.