A few mentionables from questions raised:

+ Temperature is kept at 74 F
+ Doubtful it's tail rot.  My wife and I have witnessed the small guppy attacking the larger one's tail.
+ I test the water quality weekly with a 5-in-1 test strip.  Everything has looked good since we set up the tank.  The only thing I've had concerns about was the water is tested as "hard", but the fish store said that doesn't matter as much with the fish I currently have.
+ We don't really change the water that often.  Only about 25% per month (plus a little extra if too much evaporates out).
+ I feed the fish once a day, pinch of flake and one veg tablet every other day.  The platy and the aggro guppy are the only two that eat on the tablet.
+ All fish in my tank are male, including the platy, the neons, and the two guppies.
+ Also I didn't mention it before, but the platy is about 2", the larger guppy 1-1/2", and the smaller guppy 1".  All the neons are less than an inch.  The smaller guppy is literally picking on someone twice his size.

I will try the water change tomorrow and again this weekend.  We're actually planning to buy 4-5 female guppies later this month.  We just recently purchased the neons and we try to keep new fish down to once every two months to make sure the tank settles in between.

Thanks again