16mm supported rails will handle 80mm box section easily so wouldn't be bothered there.

Dump the link pallet idea as it will only cause you grief. No point spoiling machine for little extra piece of box section costs.
80mm or 100mm box with with few well placed braces will be fine for plasma machine.
Regards the overhang then if you talking about using for Rotary type axis hanging off the side then yes no problem.

But if your talking about actually engraving on separate table kind of thing then I'd say don't waste your time and just build another machine because engraver and plasma are two very different animals.
Engraving machine needs to be far stiffer and accurate to work well and R&P just isn't going to cut if any real fine engraving type work is to be done.