As stated, you can't always stop the aggression with additional fish but you can by removing the aggressor. That said, in order to have a great experience in fish keeping, it's best to have a good understanding of what you want to eventually have in the tank when fully stocked ( and if they will live peacefully together), the proper order in which to place them in the tank, knowledge of their basic or special needs and proper maintenance of the aquarium. All these things together usually end up with a great experience in fish keeping. Missing 1 or more of these steps usually winds up with bad experiences. Knowledge is everything!
One thing you need to remember is that these are not human beings and as such, you can not use human emotions to make your decisions.
In the case of your Guppy, the proper ratio is 1 male to 2 + females so if you only had 1 guppy in the tank and it was a male, you should not have put 2 additional males in the tank. This only works when you put all 3 males in the tank at the same time and their are no females present in the tank. If the one you had was a female, you should not have put 2 males in with her. If the one you had was a male and you put 2 females in with him, you may have had a totally different result and the loss of sleep and cinnamon rolls would not have happened.
Regarding your original Tetras and Guppies, not all Tetras are a good match with Guppies and this may have been part of your original problem. Personally, I don't keep any Tetras with any Guppies since most Tetras are known to nip fins and Guppies are most designed for their nice coloration and finnage. Not a good match in my opinion.
So, if I may suggest, spend some time online or reading aquarium books to better understand your fish's behavior before you start adding more fish. Know the habits of the particular fish and not just that they are " Community" Aggressive" "Schooling", etc. Not all community fish can live peacefully together just as not all aggressive fish need to be by themselves. When properly stocked, a full fish tank can be a very enjoyable thing to spend your time watching.
Finally, please keep this in mind, fish do have a relatively short lifespan compared to other animals we keep as pets so while the loss of any pet is sad, you should not make it devastating. Even if you did everything right, it's still going to happen. You just need to do the best you can for your pets to give them the best possible life.
Hope this helps
