how big do mini mollys get and how big are there fry when they are just born?

Okay let me clarify this: There is no such thing as a mini molly. They come in 3 general species.

Peocilia Velifera, the yucatan molly, which is like a sailfin molly but can get up to 6 inches or 8cm.

Peocilia Latipinna, the true sailfin molly, which gets about 3 inches long or 6cm.

Peocilia Shenops, which is the short finned common molly we see in pet stores, which can also get about 3 inches max. Some females can even get up to 5 inches. There are varieties bred into the species such as baloon or lyretail traits, as well as some color combinations. However, there is no “mini”. Expect a fish around 2.5-3″ long.

And again, NO you cannot put a breeding trio in your 7 gallon tank.

If the water is not right from overstocking, they will not breed. Or they will and the babies will die. Overcrowding/overstocking is not something you will physically see. You have to test your water with a test kit to find out what your ammonia, nitrate, and nitrite levels are like. Babies are also very sensitive to pH changes, so you will need to test pH to make sure it isn’t changing with your water changes. If you put 3 mollies in a 7 gallon, even with a filter I can guaruntee you that they will all end up sick within a month from poor water quality (high ammonia and nitrites). You need to do a little more research on fish stocking.

Here is a guide to different types of mollies, as well as breeding FAQs:

http://www.fishlore.com/fishforum/molly/11469-moll…

Here is a site that will advise you on your tank. If you have the 6.6 gallon bookshelf tank from petco, then you have an even weaker filter. 6.5 is the closest they have to a 7 gallon tank, and I assumed you have an aqueon quietflow 10 filter which filters up to 20 gallons. So with that information I put in your info (your guppy plus guppy fry, and 3 mollies) and as you can see, it is not suitable:

http://www.aqadvisor.com/AqAdvisor.php?AquTankName…

Hope this has helped. Upgrade your tank if you want to breed.

EDIT: I wanted to add what I forgot: Assuming you have the 20 gallon filter, which is not the filter that comes with that tank, your tank is still at 127% capacity with those 5 fish. That means they will not breed, and they will likely become stunted. That means the outside of their body will stop growing while the inside continues to grow. It is a painful way to die as each of their organs slowly shuts down until they die. Any pregnant fish will hold in the babies waiting for water conditions to improve before giving birth. In such a small tank stocked that way, they will not ever give birth, and they will eventually die from the babies growing too big inside of her. If for some reason she did give birth, they would die within a few hours from ammonia levels of the tank. Fry of any fish are VERY sensitive to ammonia levels, which is why you should not consider breeding until your tank has been well cycled and you know the size of the tank can fit the number of fish you are hoping to add.

Here are what baby mollies look like in size:

https://www.youtube.com/watch?v=7rklSZD0aY8