Hi - sorry to hear about your guppies. I've got an injured guppy in my hospital tank too (injured by his own kind, sadly).
Tails do usually grow back, unless they've been injured from the root like my male cherry barb. However, that doesn't necessarily mean the fish can't survive happily, although he'd look really odd.
The time taken varies according to the severity of the injury and the size of the tail - my betta's fins are slowly growing back after he was attacked by some tetras (who probably mistook him for some giant fly wings), but it's going to take several weeks.
Melafix is soluble tea-tree oil (you may have heard of tea-tree oil being used in humans for fungal infections and acne? ). It's a relatively mild anti-bacterial, anti-fungal product that can help prevent infections occurring in these kinds of situations. It is safe for your filter and won't do any harm, but remember to take any carbon out of your filter if you have any in there.
Depending on what other species you have in there, salt can be useful to aid healing.