Bianca, the plastic anchors seen to be getting cheaper and worse as we go along. I have seen some from China that were pretty shoddy. They would probably be fine in drywall but not for your application. The best thing that I can tell you, I think, is to find a good hardware store or home center and tell them what you have to do. I know that this will be tough because your choice of people to ask will be the 16 year old kid who hasn't got a clue and the grumpy old guy that knows but guards his knowledge with his life. One game plan will be to go to one store and ask what they think then go to another store and ask that they think about that suggestion. It may take a bit longer, but it sounds like you have enough knowledge to make a good decision when you hear facts and good advise.
Personally, the soft lead ones have worked pretty well in bigger jobs for me but the lead producers do not seem to be making many of these today.