If you 'read' this or that on-line about growing, make sure the person is showing their wares. I look to see photos showing successful product and then read or ask questions. I'm not a commercial or medical grower but I do have a number of soil grow logs on this site and probably some photos in the album section.

As far as using guano... Yes, it can be a strong organic supplement and it could be misapplied but I don't agree with some of the opinions you've received. It's the salt nutrients that get in the soil and are harmful to the beneficial microbes, not guano.
As far as using guano, it can be added to the soil initially, it can be used as a top dressing or in a tea. Look for tea recipes from successful growers. Also, there are different types of guano with different NPK values which make one type preferred over another for a particular purpose.