Hi Puddin,
I had the same problem back when I had my first tank about 10-12 years ago. I had a 10 gallon tank with 3 male guppies, a female black molly and another fish (can't remember...possibly a catfish of some sort) The guppies would chase the molly. A friend who was a fish freak told me that it was possible that they were chasing her because they wanted to mate (apparently they do, even though it's 2 different species) I installed some really tall bushy plants in the tank for the molly to hide. Unfortunately I can't tell you what happened after that. It's too long ago, and I was only in 5th or 6th grade!!
Since 1999 I have a 29 gallon tropical tank with a group of lemon tetras (unkillable and very long lived) and a clown loach (I know it's too big for the tank, but the petshop who sold it to us knew nothing about them

, and neither did I )