FWIW, I prefer to place the card wads in the shot cup before dropping the shot, resulting in a shot column of the correct height for a proper crimp and chance of the card wad interfering with the pattern integrity. I use 16 gauge wads in the light 10 gauge loads I'm using for my older side by sides and 28 gauge wads for my 16 gauge reloads.
Part of the reason I'm doing this at all is that shot cups for light loads in 10 gauge and 16 gauge have been particularly difficult to find. I'm shooting 1-1/8 ounce loads in a 7 pound 3 ounce Darne 10 gauge and 7/8 ounce loads in my two 16s, but of which are pre-WWII and one of which has 2-1/2 inch chambers. So far, I've been delighted with the results. The plastic wads are nominally for 1-5/8 ounces of shot in the 10 gauge and 1-1/8 ounces of shot in the 16. Depending on the powder and the wad brand, I've needed either 2 or 3 of the 1/8 inch thick card wads to get the appropriate shot column height.
I purchased the card wads from Ballistic Products, which is where I'd guess you got your overshot wads as well.
Hope this helps.