Hi and welcome to the forum
In the wild, female livebearers hang out in large groups consisting of between 20 & 50 (but sometimes up to 100 or more) females. These groups have a pecking order with the biggest most dominant female ruling the group and she has a group of girlfriends who back her up. All the other females live in the group but are lower down the pecking order.
The groups of females move around rivers and waterways looking for food and places to hang out. As the groups move around a few males follow the group and try to breed with any females.
In the confines of an aquarium, the males will constantly harass the females and try to breed with them. This puts undue stress on the females and if there are too many males constantly pestering the females, the females can get sick and die.
It is preferable to keep livebearers in single sex tanks (either male or female but not both sexes together). If you want a group of males and females then have 1 male and at least 6 females (preferably 10 or more females per male).
Female livebearers can carry up to 6 sperm packets from breeding with males and they use 1 sperm packet to fertilise each batch of eggs. The gestation period (from the time she fertilises the eggs to when she gives birth to free swimming babies) is about 1 month. After which she will fertilise another batch of eggs using another sperm packet. This allows female livebearers to produce young about once a month for up to 6 months without any males being present.
If you want to breed livebearers then have a tank with females and let them give birth and use up all the sperm packets they are carrying in their body. Give them a few months without being pregnant and then add a male to the tank for a week before moving him out, or move the female/s into a tank with a male for a week and let them breed. Then move the females back into their own tank.
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Your female would have bred with different males at the petshop and she will be carrying sperm packets from different males. This is why the young don't look like the parents.
There is also the chance of some young looking like their ancestors, which could be any colour.
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Baby fish need to be fed at least 3 times a day and preferably 5 times a day. You feed them as much food as they can eat so it they look like pregnant guppies.
The best food for them is newly hatched brineshrimp, then microworms and other small foods. You can add dry powder foods too but brineshrimp is best. The following link has information on foods for baby fish, including a section on brineshrimp. You can watch videos on YouTube that show you how to make brineshrimp hatcheries if you want.
http://www.fishforums.net/threads/back-to-basics-when-breeding-fish.448304/
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You want the tank water warm, around 28C (82F) to grow them fast. They grow slower in cooler water and 28C is the optimum for growing these types of baby fish.
You should do daily water changes and gravel clean the substrate to compensate for the extra food going into the tank. I do a 50-75% water change and gravel clean the substrate every day on my fry rearing tanks.
If you feed them well, keep the water clean and warm, the babies should be mature in about 3-4 months.
Fish with genetic defects might grow slower than normal healthy fry.
