I don't have plans for it, and no it's not necessary but it makes things easier.
Regarding the bolt, it depends on not just the size, but also the strength of the bolt, and also the strength of the insert. I would consider ordering bow specific hardware. It would be a shame for it to fail after putting in all that work.
If I remember correctly, the bolts are around 5/16" which is pretty close to 8mm.