First, mystery snails can grow quite large, so it's important to make sure you have a tank that is large enough to accommodate both the guppies and the snails. A good rule of thumb is to allow for 1 gallon of water per guppy and 1 gallon of water per adult mystery snail.
Second, mystery snails are scavengers, and they will eat any food that they can find. This can sometimes include baby guppies. To avoid this, you can feed the guppies and snails at separate times, and you can also make sure to remove any uneaten food from the tank.
Finally, mystery snails can produce a lot of waste, so it's important to make sure you have a good filtration system in your tank. A good filter will help to keep the water clean and prevent the build-up of waste that could harm your guppies.
If you keep these things in mind, you should be able to successfully house baby guppies and mystery snails together.