If you are worried about the fry getting sucked up, be very worried that they first don't get eaten by the other fish.
If you want to save the fry, I suggest that you either remove the pregnant guppy when you can see the eyes of the fry through the mothers skin. Place her in a seperate tank. OR buy a special breeding trap and place her in there until she has her young and then she can be returned back to the main tank.