Statistics on a booster pack (to make my own re-packs)
There is no hard guarantee that a booster will contain X creatures, or X sorceries, or X red cards.

However, the contents of a booster aren't completely random either. Instead, the commons in a booster are heavily influenced by the set's common print runs.

Print runs are easiest to explain by example, so let's look at the print runs for Born of the Gods, a small set. The commons in the set are divided into two lists, with each common showing up in only one list (but appearing twice in that list).

A normal Born of the Gods booster contains 10 commons, made up of 4–6 commons from list A and 4–6 commons from list B. This means that certain combinations of commons are likely to occur together, and other combinations of commons can never occur together.

Now, all of this is probably too much of a hassle to be worth simulating when building your own repacks. I would probably just add 10 random commons from your pool. But print runs are the basis of how real boosters work, both in paper and online.