I would try to get the adults out so the young can have the tank to themself.
As mentioned lots of food and lots of water changes. Fish release a hormone to prevent other fish around them growing. In a river it doesn't matter too much because it gets diluted very quickly. In the confines of an aquarium the hormone can build up and cause stunted growth. Regular partial water changes dilute the hormone and encourage rapid growth. I do daily 50% water changes on my fry tanks.
When the livebearers are new born you feed them on newly hatched Brineshrimp. You have enough Brineshrimp in their tank so the fry eat and are full. Then you add some more Brineshrimp. Over the next few hours the fry will eat the remaining shrimp. Do this at least twice a day but preferably three times a day. The fry have to stay fat to grow rapidly. And it is most important they are kept fat during the first couple of months as this is when they do most of their growing.
As soon as the fry are big enough you start adding other foods. Micro worms can be offered from day one but grindle and small white worms can be offered when the fry are about 1/2 inch long. You can also feed finely chopped prawn and fish every meal.
Try not to rely on flake or pellet foods as they don't do much to encourage good growth. If used in addition to other foods they can be helpful but on their own they are not very good.
Keep the water warm, about 28C. The warmer water will speed up the fish's metabolism and they can grow faster. They will also digest their food faster so you have to make sure they are kept well fed. The warmer water will also encourage bacteria to grow faster so you must maintain good hygiene by doing regular gravel cleans and water changes.
Under ideal conditions livebearers should be mature, sexable and saleable at around 3 months of age.
