I think these are too small for adult G1. The original washers appear to have a 6 mm hole, so twice as big as these (btw if you check the images, there's one which has the measurements in inches!) Can you find any plastic ones in size M6? Many people also like to make the tails on their customs a little thicker. I've occasionally tried to use washers harvested from ponies but I usually struggle pulling the tail through them...
Zip ties are always an option, or you could even use string. I usually add a dab of glue as well, just to make sure no hairs are pulled when combing the tail, but this is totally optional.