The guppies, can become aggressive in small groups which is why it is recommended to have at least 4-5 :) lowers the chance of the alpha guppy singling out another!
I've had no problems with one of my males, in a group of fancy guppies (and one endler o_O ). And they were in a 10... I think I had 4 guppies and the betta. Everyone was fine and there were no casualties as long as I did the proper water changes.... I love how the guppies I had took in Spartan as a giant guppy -.-; weirdos... anywho, I do not know of the glofish, BUT, it is reall good they get along.
I am wondering if your fish are dying because the tank is not cycled? Cycling, is done (usually with or without fish, this version WITHOUT) by letting the tank run for 4-6 weeks, adding a small amount of fish food as "ammonia" supply. Fish cycle usually uses feeder fish, for that time frame, and hopefully the fish doesn't die.
nice fish though :)