Guppies have hierarchies! The aggression you’re experiencing is probably just them figuring out the pecking order and will calm down when it’s figured out! The tails DO grow back- it just takes time. Clean water helps prevent infection. The more male guppies you have the less the injuries there will be, simply because the aggression has a wider distribution. Five male guppies is a good minimum to have. Also make sure to check your decorations to make sure there’s nothing sharp enough to tear their fins