Tapco S5 | "massively constructed from 5/8-inch MDF"
So, depending on the thickness of the base bracket you plan to use, a 3/4" screw should work OK
If it were me, to counter the vibration that is inherent in speakers, I would apply glue between bracket and speaker before screwing the bracket to the bottom of the speaker
it will work but it may change the sound you get from them as well... I agree put something between the wood and metal, perhaps some sort of rubber material
Instead of heavy light stands, why not go for proper speaker stands? These will almost certainly come with a 35mm mount at the top so your on-speaker bracket would have to fit this size. Such things are available commercially though I can't comment on the quality.
I'd also put something between your plate and the speaker--in the past for similar purposes I've cut up computer mouse mats and screwed through them.