darkxside - When adding clothing to characters, you need to remove the underlying polygons. So let's say you start with a man wearing boots, pants and T-shirt, all modelled as part of the model, not layers. Then you want to add a jacket. You need to have a system that masks everything under the jacket, and "deletes" it. It is not really deleted, the mask just makes that the polygons and textures in that masked area are not loaded, keeping your polygon count down. When you add a helmet, you do the same, a mask deletes the polygons you can't see under the helmet. Every layer you add needs to mask the layers below it. Some programs, like Fuse, have a system built in for doing this, but sometimes you have to make your own system.
That being said, the DAZ3D models are really not suitable for games, now matter how much you crunch the polygons. I worked with DAZ and Poser models for years and they are not "fixable" for game use, I know because I designed some of them. To put it in context, the eye object (which is just a sphere) of their main model, Victoria, has more polygons than the Ethan character that comes with Unity. No matter how much you crunch that you are not going to get useable results. Last time I worked with one of their characters, fully clothed, the polygon count was close to 300 000, while the average Unity model will have about 10 000 to 15 000 polygons.