not sure if this helps, but I know certain manufactures use only the #6 screws and alot of the newer games only have #8's. Some have both like yours, guess what I'm saying is it probably doesn't really matter as long as they are the same length. .... Well that and if the wood is thick enough at that point (not like a wood rail or something that could split easily without predrilling).

If a playfield did use both, the larger ones probably go into places with lots of vibration. ie pop bumpers, trough etc.. the main concern is making sure the screw has something grab onto when you put it in. if it's still loose after tightening put some wood glue on some toothpick and break them off in the hole. smooth it out and predrill if needed.

u could always put the smaller ones first and if they are loose that's probably where the number 8's went.
...or it could just be a stripped 6