I am not convinced that a seal coat adds weight.

The timber is going to absorb a certain amount of epoxy. It will either happen while you are applying the seal coat or while you are wetting out the glass. If a section of the timber has a grain pattern that makes it absorb a bit more epoxy than the rest of the timber, then starving the glass of epoxy in that spot is a possibility. Applying enough epoxy to be certain that there will be no starved spots would add more weight than a seal coat.

If you don't plan to bounce your kayak off rocks, a single layer of 4oz should be enough. I have sea kayaks that are almost a decade old with just a single layer of 4oz inside and out. I have the benefit of usually being able to land and launch on sand, so the hulls have suffered no damage at all.

I don't believe that there is enough difference between bead and cove and beveled strips to require the use of a different weight of glass. Glass that is suitable for a bead and cove kayak will be perfectly adequate for a beveled kayak.