Here's some thoughts on your questions - First, when you make the pierogis, don't make the dough rolled out too thin, because that's a good way to make them break apart and empty out in the boiling water. And don't have the water boiling too hard, just a slow boil works good.

I realized years ago when I first started making them, that when you remove them from the boiling water, you need to dry them out just a bit first, before stacking them for freezing or storing in the refrigerator. I put out several clean kitchen towels and place each pierogi on them, single layers for about 20 minutes or so. In the meantime, I take several sweet onions, quartered and slice thin and cook them slowly in a good amount of vegetable oil until they are translucent and golden colored. Don't cook them too fast, you don't want them to brown. You just want the flavors to blend into the oil.

After the pierogis have dried out somewhat on the towels, I transfer them to a big bowl, where I layer them alternating with the oil and onion mixture. You can store them nicely that way in the refrigerator until ready to use. I always re-cook them in some butter or oil, because we like the crispy, served with sour cream and the oil/onion on the side.

The other way to do them ahead of time is to freeze them after they've been boiled. Dry them as above, then place them on a cookie sheet single layers, cover lightly and freeze them. Then remove them to a ziplock baggie for freezer storage.

Hope this helps a little. Mine always stuck until an older relative told me about the drying thing, and that has solved the problem for me.

Enjoy your buffet and I hope everyone realizes how much work will go into making pierogis for 50 people!!