The reason Bio-Spira doesn't work for everyone, is because it's bacteria. Bacteria needs to be chilled to survive. When it is trucked to stores, if often gets to extreme temperatures in there. Therefore, it's very hard for the beneficial bacteria to live.
Also, while adding Guppies slowly DOES work, I've found it better to put them all in together, or near the same time. If you do it slowly, you will have groups of 2 and 3...and one Guppy is going to get picked on a lot during that time. I can almost guarantee that from my experience. Yes, even if you have calm males..one will get some chasing, and occasional nipping. When you add them all together, they can stake out their territories, and find hiding places. And they usually tend to accept more that they are a group, rather then fighting the new male Guppy that comes in to their territory each week. And the larger the group, the better.
Now, I'm not saying Toxis's idea doesn't work. It will. I have done it. But from my experience...when you add them all around the same time, there's less nipping, torn fins, and chasing. Of course, there will be SOME...they are male Guppies, after all.
But personally...I can't see all those male Guppies in a 10 gallon ...They won't have much room to get away from each other. But that's just me.