Yikes! I love yoyos, but yes, they're not easy, need a huge tank and the right numbers or they can be devils. An employee in my fish store told me that his yoyos ganged up and killed one of his oscars
I'm sorry to hear you've had such a rough time of it. It goes like that sometimes, sadly. I went through a bad few months recently too, where it seemed to be one thing after another, but now I seem to be out the other side *touch wood* and things are breeding and thriving again. You'll turn that corner again too.
Since you transferred everything including the filter, I should have thought the 10 gallon could support ten guppies, so this is a bit of a mystery I'm afraid. It's very late here and I have to go to bed too, but for now, keep on top of the water changes, and that should hopefully hold things over while we all put our heads together and figure it out!